Developing synthetic strategies for ligand free noble metal nanostructures

... values 82 6.3.3 Proposed mechanism for the formation of ligand- free nanorattles 84 6.3.4 Effect of nanocavity of mHSS on the formation of ligand- free nanorattles 86 6.4 Conclusion ... developed for tuning the size of ligand- free Au nanoparticles In this method, mesoporous hollow silica shells were employed as nanoreactors for tuning size and shape of noble metal nanostructures. .. rate inside the cavity of mHSS for the formation of yolk-shell nanoparticles The formation of ligand- free YSNs can be tuned simply by varying the pH of the noble metal precursor aqueous solution

DEVELOPING SYNTHETIC STRATEGIES FOR LIGAND-FREE NOBLE METAL NANOSTRUCTURES SHAIK FIRDOZ (M.Tech, IIT-Roorkee, India) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2014 DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. SHAIK FIRDOZ 19 Dec 2014 Acknowledgement Acknowledgement Firstly, I would like to express my sincere heartfelt gratitude to my supervisor, Asst. Prof. Lu Xianmao, for his suggestions, invaluable guidance and continuous support given to me during my entire PhD candidature. In the same, I would like to extend my sincere thanks to the Department of Chemical and Biomolecular Engineering for offering me a NUS research scholarship, which helps me a lot to complete my PhD studies successfully without any financial hurdles during my PhD candidature. I would like to express my thanks to my lab mates Dr. Zhang Weiqing, Dr. Niu Wenxin, Dr. Sun Zhipeng, Dr. Chen Ningping, Dr. Han Hui, Dr. Guo Chunxian, Dr. Chen Shaofeng, Mr. Ton Tran, Ms. Zhao Dieling, Mr. Zhao Qipeng, Mr. Chen Shucheng and Ms. Gamze Yilmaz for their valuable discussions and the good fun we shared in the lab. I would like to thank Mr. Chia Phai Ann, Mr. Liu Zhicheng, Mr. Mao Ning, Dr. Yuan Zeliang, Dr. Yang Liming, Mr. Tan Evan Stephen, Mr. Ang Wee Siong, Ms. Li Fengmei, Ms. Li Xiang and Ms. Samantha for their kind support and help during my PhD candidature. I would like to thank specially my friends Mr. Karthik, Mr. Akshay, Mr. Dara Nuthan and Mr. Sai Naresh reddy for their kind help, suggestions and tons of fun given to me and make my stay very pleasant and happy. Finally, I would like to express my deepest love to my beloved parents, parent-in-law and brother-in-law Mr. Shaik Abid, for their kind support. Last but not least, my sincere and special thanks given to my wife Ms. Shaik Asma for her un-conditional support and help i Acknowledgement during my study. It is not exaggeration to say that I could not able to complete my PhD work without her support and love. I would like to extent my sincere thanks to all my friends and well-wishers who directly or indirectly help me to finish my PhD work. ii Table of Contents Table of Contents Acknowledgement ............................................................................... i Summary .......................................................................................... viii List of Abbreviations ......................................................................... xi List of Figures .................................................................................. xiii List of Tables .................................................................................... xxi Chapter 1. Introduction ......................................................................1 1.1 Background .................................................................................................. 1 1.2 Objectives .................................................................................................... 2 1.3 Organization of thesis .................................................................................. 3 Chapter 2. Literature Review ............................................................4 2.1 Core-shell nanoparticles .............................................................................. 4 2.2 Rattle–type hollow structures (Yolk-shell / Nanorattles)............................ 5 2.3 Methodologies for the fabrication of noble metal (M@SiO2) YSNs .......... 6 2.3.1 Synthetic approaches .................................................................................................... 6 2.3.2 Selective etching or dissolution method ...................................................................... 7 2.3.3 Pre-shell method or Ship-in-bottle method ................................................................ 14 2.3.4 Template free methods ............................................................................................... 18 2.3.5 Galvanic replacement method .................................................................................... 19 2.3.6 One pot method .......................................................................................................... 19 2.4 Applications of Yolk-shell noble metal nanoparticles .............................. 22 iii Table of Contents 2.4.1 Yolk-shell nanoparticles as nanoreactors ................................................................... 22 2.4.2 Yolk-shell nanoparticles as drug delivery carriers ..................................................... 26 2.4.3 Yolk-shell nanoparticles for lithium-ion batteries ..................................................... 28 2.5 Our Proposed Method................................................................................ 29 Chapter 3. Experimental Section .....................................................31 3.1 Method and materials ................................................................................ 31 3.2 Solution preparation .................................................................................. 32 3.3 Procedures ................................................................................................. 32 3.3.1 Synthesis of mesoporous hollow silica shells of size 100 nm and 230 nm (mHSS-100 & mHSS-230)...................................................................................................................... 32 3.3.2 Synthesis of silica spheres (SiO2) .............................................................................. 33 3.3.3 Synthesis of ligand-free Au@SiO2 nanorattles by thermal method ........................... 34 3.3.4 Catalytic reduction of 4-nitrophenol by Au@SiO2 nanorattles ................................. 35 3.4 Synthesis of ligand-free Au nanoplates by photochemical reduction method ............................................................................................................. 36 3.4.1 Synthesis of spherical gold yolk-shell nanoparticles in absence of Ag+ ions (Au@mHSS) ....................................................................................................................... 36 3.4.2 Synthesis of Au triangular nanoplates in the presence of Ag+ ions ........................... 37 3.4.3 Etching of Au triangular yolk-shell nanoplates with HF ........................................... 37 3.5 Synthesis of ligand-free M@SiO2 (M= Au, Ag, Pt and Pd) nanorattles by using mHSS as smart nanoreactors ................................................................. 38 3.5.1 Synthesis of Ag@mHSS yolk-shell nanoparticles ..................................................... 38 3.5.2 Synthesis of Ag@mHSS yolk-shell nanoparticles with different pH values ............. 38 3.5.3 Synthesis of Au@mHSS yolk-shell nanoparticles ..................................................... 38 iv Table of Contents 3.5.4 Synthesis of Pd@mHSS yolk-shell nanoparticles ..................................................... 39 3.5.5 Synthesis of Pt@mHSS yolk-shell nanoparticles ...................................................... 39 3.6 Characterization Methods .......................................................................... 39 3.6.1 Ultraviolet-visible spectrophotometer (UV-Vis) ....................................................... 39 3.6.2 X-ray photoelectron spectroscopy (XPS)................................................................... 39 3.6.3 Inductively coupled plasma mass spectrometry (ICP-MS) ........................................ 40 3.6.4 Brunauer-Emmett-Teller (BET) measurements for mHSS-100 and mHSS-230 ....... 40 3.6.5 Zeta-Potential measurements ..................................................................................... 40 3.6.6 FT-IR measurement for mHSS-100 ........................................................................... 41 3.6.7 Mass spectrum analysis of different noble metal precursor’s solutions..................... 41 3.6.8 Scanning electron microscopy (SEM) ....................................................................... 41 3.6.9 Transmission electron microscopy (TEM) ................................................................. 41 Chapter 4. Volume-confined Synthesis of Ligand-free Gold Nanoparticles with Tailored Sizes for Enhanced Catalytic Activity .............................................................................................................42 4.1 Introduction ............................................................................................... 42 4.2 Results and discussion ............................................................................... 45 4.2.1 Synthesis of mesoporous hollow silica shells (mHSS) .............................................. 45 4.2.2 Synthesis of Au@SiO2 nanorattles using mHSS-230 ................................................ 46 4.2.3 Characterizations of Au@mHSS nanorattles ............................................................. 49 4.2.4 Synthesis of Au@SiO2 nanorattles using mHSS-100 ................................................ 52 4.2.5 Catalytic activities of Au@SiO2 nanorattles for reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in presence of excess NaBH4 ......................................................... 54 4.3 Conclusion ................................................................................................. 60 v Table of Contents Chapter 5. Synthesis of Ligand-free Au Triangular Nanoplates.61 5.1 Introduction ............................................................................................... 61 5.2 Results and discussion ............................................................................... 64 5.2.1 Synthesis of Au triangular nanoplate inside mHSS ................................................... 64 5.2.2 Characterizations of Au triangular nanoplates ........................................................... 65 5.2.3 Synthesis of spherical gold nanoparticles inside mHSS in the absence of Ag+ ions.. 68 5.2.4 Effect of chloride ions on the formation of Au triangular nanoplates ....................... 70 5.2.5 Proposed mechanism for the growth of Au triangular nanoplate inside mHSS ........ 70 5.3 Conclusion ................................................................................................. 73 Chapter 6. Synthesis of M@SiO2 (M = Ag, Au, Pd, Pt) Yolk-Shell Nanoparticles .....................................................................................74 6.1 Introduction ............................................................................................... 74 6.2 Results and discussion ............................................................................... 76 6.2.1 Synthesis of ligand-free M@SiO2 nanorattles (M = Ag, Au, Pd & Pt) using mHSS 76 6.2.2 Characterizations of ligand-free M@SiO2 nanorattles (M = Ag, Au, Pd & Pt) ......... 78 6.3 Growth mechanism for the formation of ligand-free nanorattles.............. 79 6.3.1 Synthesis of Ag@mHSS nanorattles at different time periods .................................. 79 6.3.2 Synthesis of Ag@mHSS nanorattles at different pH values ...................................... 82 6.3.3 Proposed mechanism for the formation of ligand-free nanorattles ............................ 84 6.3.4 Effect of nanocavity of mHSS on the formation of ligand-free nanorattles .............. 86 6.4 Conclusion ................................................................................................. 87 Chapter 7. Conclusions and Future Work .....................................89 7.1 Conclusions ............................................................................................... 89 vi Table of Contents 7.2 Future Work ............................................................................................... 93 References .........................................................................................96 Annexure ......................................................................................... 115 List of Publications ........................................................................................ 115 vii Summary Summary As an important class of nanomaterials, noble metal nanostructures (NMNs) have attracted significant attention because of their wide variety of applications in catalysis, photonics, sensing and medicine. It is well known that the catalytic and optical properties of NMNs can be effectively tailored by tuning their size and shape. Therefore, a number of synthetic routes have been developed for morphology-controlled synthesis of NMNs. Among these synthetic methods, wet chemical synthesis is probably the most powerful approach to the preparation of noble metal nanostructures in large quantity with controlled shapes, sizes, and compositions. To achieve control over shape and size during synthesis, capping ligands such as polymers, surfactants, dendrimers, or small organic molecules are generally employed. However, the presence of ligands on the surface of NMNs may drastically affect their catalytic activity, stability, and suitability for biological applications. Especially for catalysis, bare NMNs free of capping ligands are highly desirable. But without capping ligands, it is difficult to tailor the size and morphology of the nanoparticles since they would all grow into the most thermodynamically stable form. In addition, ligand-free NMNs are highly unstable and easily aggregate in solution. Therefore, the objective of the current research is to design synthetic methods for ligandfree NMNs with controllable size, shape, composition and exposed facets. Firstly, we developed a volume-confined method to tune the size of gold nanoparticles inside the cavity of mesoporous hollow silica shells (mHSS) without using any organic capping ligands. In this method, mHSS with an average sizes of 100 ± 2.3 nm (mHSS-100, pore volume: 1.321 cm3/g) and 230 ± 4.2 nm (mHSS-230, pore volume: 0.215 cm3/g) were synthesized by using viii Summary a modified Stober’s method. mHSS were infiltrated with HAuCl4 solution followed by centrifugation to separate impregnated mHSS with AuCl4- ions from the bulk solution. A simple heating process was then employed to convert the Au precursor solution trapped in the cavity of mHSS into spherical Au nanoparticles. The sizes of the Au nanoparticles were tailored by simply varying the concentration of Au precursor solution loaded inside the cavity of mHSS, or by using mHSS with different cavity volumes. The resulting ligand-free Au nanoparticles exhibited much enhanced catalytic activity towards the reduction of 4nitrophenol to 4-aminophenol with NaBH4 compared to citric acid-capped Au nanoparticles of the same size. Next, a photochemical reduction method together with the introduction of Ag+ ions was developed to grow ligand-free Au triangular nanoplates inside the cavity of mHSS. In this case, mHSS were soaked in a mixture of HAuCl4, ethanol and AgNO3 solution. After centrifugation, the samples were exposed to UV light. The ethanol molecules trapped inside the mHSS generated radicals which acted as reducing agent to reduce AuCl4-. The resulting Au atoms nucleated inside the mHSS and grew into nanoparticles. The shape of the gold nanoparticles was successfully tuned from spheres to triangular nanoplates by varying the ratio of [AuCl4-]:[Ag+]. It is believed that the presence of Ag+ ions can alter the reduction kinetics of AuCl4- and facilitate the development of twinned seeds, which gradually develop into Au triangular nanoplates inside the nanocavity of mHSS. The effect of Ag+ and ethanol for the formation of Au triangular nanoplates was examined. Finally, we extended the synthesis of ligand-free nanoparticles to other noble metals including Ag, Pd, Pt which cannot be obtained inside of mHSS by using simple heating methods. In this study, the precursor solutions of different noble metals were loaded inside ix Summary the mHSS first. The growth of nanoparticles was achieved by tuning deprotonated species (≡Si-O-) present on the mHSS with different pH range of solution. It was found that pH > 4 is required for the synthesis of ligand-free Ag, Pd, Pt, as well as Au nanoparticles, while at pH 4, deprotonated species (≡Si-O-) present on the mHSS surface can act as reducing agent for the conversion of noble metal precursor ions into metal atoms. The nanocavity of mHSS also plays a critical role to lower the critical radius of nucleation for the growth of nanoparticles inside the cavity of mHSS. x List of Abbreviations List of Abbreviations YSNs Yolk-shell nanoparticles mHSS Mesoporous hollow silica shells NMNs Noble metal nanostructures NPs Nanoparticles SiO2 Silicon dioxide M@SiO2 M = Ag, Au, Pd, Pt PS-124 Polystyrene beads (124 nm) PS-266 Polystyrene beads (266 nm) Au Gold Ag Silver Pd Palladium Pt Platinum AgNO3 Silver nitrate H2PtCl6.xH2O Chloroplatinic acid hydrate HAuCl4∙3H2O Gold(III) chloride trihydrate Na2PdCl4 Sodium tetrachloropalladate NaOH Sodium hydroxide HNO3 Nitric acid DI H2O Deionized water NH3.H2O Ammonia solution VTMS Vinyltrimethoxysilane HF Hydrofluoric acid PBzMA Poly(benzyl methacrylate) NaCl Sodium chloride PEMs Polyelectrolyte multilayers MF Melamine-formaldehyde HCl Hydrochloride acid TEOS Tetra ethyl orthosilicate TSD N-3-(trimethoxysilyl) propyl ethylenediamine xi List of Abbreviations PVP Poly(N-vinyl-2-pyrrolidone) SPR Surface plasmon resonance UV Ultraviolet CTAB Hexadecyltrimethylammonium bromide SERS Surface enhanced Raman scattering PS-co-PMAA Poly(styrene-co-methylacrylic acid) TOF Turn over frequency PEG Polyethylene glycol DOX Doxorubicin IBU ibuprofen MRI Magnetic resonance imaging PEO-PPO-PEO Polymer(poly(ethyleneoxide)-poly(propyleneoxide)poly(ethylene) SC EDX Sodium citrate oxide), Energy dispersive X-ray spectroscopy FESEM Field Emission Scanning Electron Microscopy TEM Transmission electron microscopy HRTEM High-Resolution Transmission Electron Microscopy UV-Vis Ultraviolet-Visible XPS X-ray Photoelectron Spectroscopy ICP-MS Inductively coupled plasma mass spectrometry ECSAs Electrochemically activated surface areas BET Brunauer-Emmett-Teller SAED Selected Area Electron Diffraction FT-IR Fourier transform infrared spectroscopy 4-AP 4-aminophenol 4-NP 4-nitrophenol xii List of Figures List of Figures Figure 2.1 Different core/shell nanoparticles: (a) spherical core/shell nanoparticles; (b) hexagonal core/shell nanoparticles; (c) multiple small core materials coated by single shell material; (d) nanomatryushka material; (e) movable core within hollow shell material. Reprinted with permission from reference.7 .............................................................................. 5 Figure 2.2 Schematic illustration of etching strategies for preparing YSNs. Reprinted with permission from reference.20...................................................................................................... 6 Figure 2.3 (A, B) Backscattering SEM and (C, D) TEM images of Au@SiO2@PBzMA particles before (A, C) and after (B, D) HF etching. (E, F) TEM images of Au@SiO2@PBzMA synthesized at different time periods of polymerization: (E) 3 h and (F) 6 h. Reprinted with permission from reference.22 ...................................................................... 8 Figure 2.4 TEM images of (A) PS-co-P4VP microspheres, (B) Au/PS-co-P4VP microspheres, (C, D) Au/PS-co-P4VP@HMSM microspheres, and (E, F) Au@HMSM yolkshell microspheres. Reprinted with permission from reference.23 ............................................. 9 Figure 2.5 TEM images of (A) Au-decorated PS nanospheres, (B) PS@Au@silica colloids, and (C) hollow silica spheres with multi Au nanoparticles after burning off PS templates. (D) Backscattering SEM (inset, normal SEM) image of multicore hollow silica spheres. Reprinted with permission from reference.24 ........................................................................... 10 Figure 2.6 (a) Schematic illustration for the synthesis of Au@mSiO2 and Fe2O3@mSiO2 YSNs; TEM images of (b) Au@SiO2@mSiO2, (c) rattle-type Au@mSiO2, (d and inset) xiii List of Figures ellipsoidal Fe2O3@SiO2@mSiO2, (e, inset: SEM image of deliberately selected broken ellipsoids) rattle-type Fe2O3@mSiO2. Reprinted with permission from reference.28 .............. 12 Figure 2.7 (a-f) TEM images of Au@SiO2 nanoreactors and (g, h) silica hollow shells. (a, b) Gold core diameters are 104±9 nm, (c, d) 67±8 nm, and (e, f) 43±7 nm. The scale bars represent 200 nm (a, c, e, and g) and 100 nm (b, d, f, h). Reprinted with permission from reference.18 ............................................................................................................................... 13 Figure 2.8 Schematic procedure for the preparation of yolk/shell nanoparticles through the ship-in-bottle method. Reprinted with permission from reference.20 ...................................... 14 Figure 2.9 TEM images of hollow silica particles (a) and Cu core rattle-type silica particles (b: 1 cycle; c: 2 cycles; d: 3 cycles). Reprinted with permission from reference.31 ................ 15 Figure 2.10 Schematic procedure for the preparation of PPy-CS hollow nanospheres containing movable Ag Cores (Ag@PPy-CS). Reprinted with permission from reference.33 16 Figure 2.11 TEM images of (a) silica nanorattles of 110 nm, (b) SRG-1, (c) SRG-2, and (d) SRG-3. Reprinted with permission from reference.34.............................................................. 17 Figure 2.12 Schematic formation of SnO2 hollow spheres inside mesoporous silica nanoreactors. Reprinted with permission from reference.35 .................................................... 17 Figure 2.13. TEM images showing the formation of hollow CoSe nanocrystals, from top-left to bottom-right: 0 s, 10 s, 20 s, 1 min, 2 min, and 30 min. The Co/Se molar ratio was 1:1. Reprinted with permission from reference.36 ........................................................................... 18 xiv List of Figures Figure 2.14 TEM images of Au@HSNs synthesized using 1 ml of HAuCl4 solution: (a-b) before calcination and after being washed several times with water; scale bars: 50 and 20 nm, respectively; (c-d) after calcination, the small clusters of Au aggregate to give larger Au nanoparticles (indicated by white arrows); scale bars: 100 and 20 nm, respectively. Reprinted with permission from reference.44............................................................................................ 20 Figure 2.15 (A) Structural illustration of an Au@oxide composite nanoreactor. (B) TEM image of Au@oxide composite nanoreactors. (C) Typical reactions tested for Au@oxide composite nanoreactors. Reprinted with permission from reference.20 ................................... 23 Figure 2.16 (A) TEM images of Au@SiO2 yolk-shell nanoreactors, and (B) Time-dependent UV-vis spectral changes of Au@SiO2 yolk/shell nanoreactors used for catalytic studies. Reprinted with permission from reference.46 ........................................................................... 25 Figure 2.17 (A) TEM images of (a) Au@SiO2, (b) Au@SiO2@ZrO2, (c) Au@hm-ZrO2, (d) Au@SiO2@TiO2, and (e) Au@hm-TiO2. (B) A comparison of catalytic activity for CO oxidation by Au@hm-TiO2 (squares) and Au@hm-ZrO2 (circles) nanoreactors calcinated at 100 °C and 300 °C respectively. Reprinted with permission from reference.49....................... 25 Figure 2.18 (A) Schematic illustration for the synthesis of Pd@mesoporous silica composite nanoreactors. (B) TEM images of Pd@mesoporous silica composite nanoreactors. (C) Typical Suzuki reaction. Reprinted with permission from reference.20................................... 26 Figure 2.19 (A) Schematic illustration for the synthesis of YSNs-PEG/FA. (B) A possible mechanism accounting for killing of MCF-7 cells by DOX-YSNs. Reprinted with permission from reference.20 ...................................................................................................................... 27 xv List of Figures Figure 2.20 TEM images of (a) Co@Au nanospheres and (b) cells exposed to Co@Au nanospheres. Reprinted with permission from reference.56 ..................................................... 28 Figure 4.1 Schematic illustration for the synthesis of ligand-free Au nanoparticles using hollow mesoporous silica shells............................................................................................... 44 Figure 4.2 TEM images of (a) PS-266@SiO2 core-shell particles, (b) 230-nm SiO2 hollow shells, and (c) SEM image of 230-nm SiO2 hollow shells....................................................... 45 Figure 4.3 N2 adsorption-desorption isotherms of (a) mHSS-100 and (c) mHSS-230; and pore size distributions of (b) mHSS-100 and (d) mHSS-230. ................................................. 46 Figure 4.4 TEM images of (a) 230-nm SiO2 hollow shells and (b-f) Au nanoparticles with diameters of 7 (Au@SiO2-7), 10 (Au@SiO2-10), 26 (Au@SiO2-26), 36 (Au@SiO2-36) and 42 nm (Au@SiO2-42) synthesized inside of the hollow shells using HAuCl4 solutions of 0.005, 0.01, 0.1, 0.25 and 0.5 M, respectively. ........................................................................ 48 Figure 4. 5 Plot of measured and calculated particle diameters vs. [HAuCl4]1/3. ................... 49 Figure 4.6 (a) HRTEM image of a single Au nanoparticle in Au@SiO2-26. (b) UV-vis extinction spectra of Au@SiO2-10, Au@SiO2-26, and Au@SiO2-36 (c, d) EDX line spectrum of a single gold nanoparticle of Au@SiO2-26. ........................................................................ 50 Figure 4.7 EDX spectrum of Au@SiO2-26. ........................................................................... 51 Figure 4.8 XPS spectra of as-synthesized Au@SiO2-26: (a) Au 4f and (b) survey scan. ...... 51 xvi List of Figures Figure 4.9 (a) SEM and (b) TEM images of 100-nm SiO2 hollow shells (mHSS-100). (c, e, g) TEM images of Au nanoparticles with diameters of 6 (Au@SiO2-6), 14 (Au@SiO2-14), 18 nm (Au@SiO2-18), synthesized inside of mHSS-100 by impregnating HAuCl4 solutions of 0.01, 0.25 and 0.5 M, respectively; (d, f, h) corresponding histograms of Au nanoparticle sizes for Au@SiO2-6, Au@SiO2-14, and Au@SiO2-18 respectively; (i) a low magnification .................................................................................................................................................. 53 Figure 4.10 Time-dependent UV-Vis absorption spectra for the reduction of 4-nitrophenol catalyzed by (a) Au@SiO2-26 and (c) Au@SC-26, respectively. (b) ln(Ct/C0) vs. time for the reduction of 4-nitrophenol catalyzed with Au@SiO2 and Au@SC nanoparticles of different sizes in the presence of excess NaBH4. (d) Conversion of 4-nitrophenol vs. time (Inset: TOF bar chart). ................................................................................................................................. 55 Figure 4.11 Time-dependent UV-Vis absorption spectra for the reduction of 4-nitrophenol catalyzed by (a) Au@SC-10, (b) Au@SiO2-10, and (c) Au@SiO2-36, respectively. ............. 56 Figure 4.12 TEM images of sodium citrate-capped Au nanoparticles: (a) Au@SC-10, (c) Au@SC-26 and (b, d) the corresponding histograms of Au nanoparticle sizes, respectively. 59 Figure 5.1 (a) Schematic illustration for the growth of ligand-free Au triangular nanoplates and nanoparticles inside silica shells under UV irradiation. (b-d) TEM images of Au nanocrystals synthesized under UV irradiation for 24 hours at different ratios of [AuCl4-]: [Ag+] -- (b) 30:1; (c) 50:1; and (d) without Ag+. (e) UV-vis absorption spectra of the Au nanocrystals.............................................................................................................................. 63 xvii List of Figures Figure 5.2 TEM images of (a) 124 nm PS beads, (b) PS-124@vinyl-SiO2, (c) mHSS, and (d) FESEM image of mHSS. ......................................................................................................... 64 Figure 5.3 EDX elemental mappings of Au (red) and Ag (green) and the spectrum for the Au nanoplates synthesized at Au:Ag = 30:1. All scale bars are 50 nm. ........................................ 65 Figure 5.4 (a) TEM image of the Au triangular nanoplates after the removal of silica shells. (b) SAED pattern of a Au triangular nanoplate. (c) HRTEM image of a Au triangular nanoplate. (d) TEM side view of a Au triangular nanoplate.................................................... 67 Figure 5.5 TEM images of Au triangular nanoplates synthesized in the presence of Ag+ ions under UV irradiation at different times: (a) 1, (b) 6, (c) 12 and (d) 24 hrs (Scale bars: 50 nm). The insets are the corresponding HRTEM images (Scale bars: 5 nm). ................................... 68 Figure 5.6 TEM images of Au@mHSS synthesized without Ag+ ions under UV light for different time periods: (a) 1 hour, (b) 6 hour, (c) 12 hour and (d) 24 hour. (All scale bars are 50 nm). ..................................................................................................................................... 69 Figure 5.7 TEM images of the samples obtained by soaking mHSS in 0.1M aqueous HAuCl 4 solution for a period of 24 hours (a) with ethanol but no UV light irradiation and (b) with UV light but without ethanol. (c) TEM image of Au@mHSS synthesized by soaking mHSS in a mixture of 0.1 M HAuCl4 (prepared with saturated NaCl solution) and ethanol but without Ag+ ions. .................................................................................................................................. 70 Figure 5.8 HRTEM images of Au seeds obtained in the presence and absence of Ag+ ions: (a) twinned and (b) single crystalline seeds respectively......................................................... 72 xviii List of Figures Figure 6.1 Schematic illustration for the synthesis of ligand-free yolk-shell nanoparticles M@mHSS (M=Ag, Au, Pd & Pt). ........................................................................................... 76 Figure 6.2 TEM images of ligand-free M@mHSS (M = Ag, Au, Pd & Pt) synthesized by soaking mHSS in respective metal precursor solutions: (a) Ag@mHSS, (b) Au@mHSS, (c) Pd@mHSS, and (d) Pt@mHSS. .............................................................................................. 77 Figure 6.3 Low magnification TEM images of (a) Ag@mHSS, (b) Au@mHSS, (c) Pd@mHSS, and (d) Pt@mHSS. .............................................................................................. 78 Figure 6.4 (a, d, g, j) HRTEM images of Ag, Au, Pd and Pt core inside of mHSS. (b, e, h, k) TEM images of M@mHSS (M= Ag, Au, Pd & Pt) and (c, f, i, l) the corresponding EDX elemental mappings of Ag, Au, Pd, Pt (red), O (blue) and Si (green), respectively. (All unmarked scale bars are 50 nm)............................................................................................... 79 Figure 6.5 TEM images of Ag@mHSS synthesized by soaking mHSS in 0.1M AgNO 3 solution for different time periods: (a) mHSS, (b) 1 min, (c) 1 hour, (d) 6 hour, (e) 12 hour and (f) 24 hour. ........................................................................................................................ 81 Figure 6.6. UV-vis absorption spectra of mHSS, AgNO3 solution aged for 24 hours, and mHSS in AgNO3 solution aged for 24 hours. ......................................................................... 82 Figure 6.7 TEM images of Ag@mHSS synthesized by soaking mHSS in 0.1M AgNO3 solution for a period of 1 hour at different pH values: (a) 1.6, (b) 2.3, (c) 3.1, (d) 4.2, (e) 5.3 and (f) 9.3. ................................................................................................................................ 83 Figure 6.8 Schematic illustration of the formation of metal core nanoparticles inside mHSS. .................................................................................................................................................. 84 xix List of Figures Figure 6.9 FT-IR spectrum of mHSS. ..................................................................................... 85 Figure 6.10 TEM images of (a, c) Au and (b, d) Ag nanoparticles formed on the surface of SiO2 spheres. ............................................................................................................................ 87 xx List of Tables List of Tables Table 3.1 Chemicals and materials ......................................................................................... 31 Table 3.2 Concentration of Au present in different YSNs measured by ICP-MS. ................. 36 Table 4.1 Comparison of the catalytic activities of Au@SiO2 nanorattles and sodium citratecapped Au nanoparticles for the reduction of 4- nitrophenol. ................................................. 58 Table 6.1. Zeta-potentials of mHSS at different pH values. ................................................... 85 xxi Chapter 1 Chapter 1. Introduction 1.1 Background As an important class of nanomaterials, noble metal nanostructures (NMNs) have attracted significant attention because of their wide variety of applications in catalysis, photonics, sensing and medicine. Particularly, the catalytic and optical properties of NMNs can be effectively tailored by tuning their size and shape. Among various synthetic methods, wet chemical synthesis is probably the most powerful approach to prepare noble metal nanostructures in large quantity and with controlled shape, size, and composition. In typical wet chemical syntheses, capping agents are employed to achieve arrested growth and control over the morphology of the resulting nanostructures. The functions of capping agents are to prevent aggregation, increase stability, and direct the shape of the nanoparticles. Using various capping agents, noble metal nanoparticles with diverse shapes such as spheres, rods, cubes, disks, wires, tubes, branched, triangular prisms and tetrahedral nanoparticles have been produced.1 But the presence of surfactants on the surface of nanoparticles may drastically affect their catalytic activity, stability in harsh conditions and usage in biological applications.2 For example, the activity of gold catalysts is detrimentally affected when strong covalent capping agents (e.g., alkanethiol molecules and phosphine complexes) are present even in minute amounts.3 Methods like laser ablation4 and bio-based approaches5-6 are currently available to synthesize ligand-free noble metal nanoparticles. However, besides tedious procedures, the difficulties in scaling up the synthesis and controlling the growth of nanoparticles have 1 Chapter 1 limited the use of these methods. In addition, bare NMNs are highly active and aggregate easily. 1.2 Objectives The current research objective is to develop synthetic methods for the growth of ligandfree NMNs with controllable size, shape, composition and exposed facets. Specially, we employ mesoporous hollow silica shell (mHSS) as the templates to form nanoparticles inside their cavity without using any capping ligands. The detailed research activities and scope of the thesis are as follows: A facile volume confined synthesis was developed for tuning the size of ligand-free Au nanoparticles. In this method, mesoporous hollow silica shells were employed as nanoreactors for tuning size and shape of noble metal nanostructures. mHSS served as nanocontainers for the impregnation of HAuCl4 solution before they were separated from the bulk solution. With a simple heating process, the Au precursor confined within the cavity of the isolated hollow shells was converted into ligand-free Au nanoparticles. The size of the Au nanoparticles can be tuned precisely by loading HAuCl4 solution of different concentrations, or by using mHSSs with different cavity volumes. With the reduction of 4-nitrophenol in presence of NaBH4 as a model reaction, we further assessed the catalytic activity of the ligand-free Au nanoparticles and found much improved performance compared to sodium citrate capped Au nanoparticles. A photochemical reduction method was introduced to tune the shapes of ligand-free gold nanoparticles inside the nanocavity of mHSS in the presence of Ag+ ions. We found that, by varying the molar ratio of [AuCl4-]: [Ag+] from 50:1 to 30:1 in the reaction, the shape of the gold nanoparticles can be tailored from spheres to triangular plates. 2 Chapter 1 Finally, we investigated a facile method for the synthesis of ligand-free noble metal M@SiO2 (M=Ag, Au, Pd, Pt) yolk-shell nanoparticles. In this study, we demonstrated that the nanocavity of mHSS plays a critical role to lower the critical radius of the nucleation and enhanced nucleation rate inside the cavity of mHSS for the formation of yolk-shell nanoparticles. The formation of ligand-free YSNs can be tuned simply by varying the pH of the noble metal precursor aqueous solution. We found that pH > 4 is required for the synthesis of yolk-shell nanoparticles; whereas, for pH < 4 nanoparticles were not obtained. This is because the deprotonated species (≡Si-O-) on mHSS surface at higher pH > 4 can act as reducing agents for the conversion of noble metal ions to nanoparticle. 1.3 Organization of thesis The thesis is composed of seven chapters. Chapter 1 introduces the general background of noble metal nanoparticles and research objectives. Chapter 2 provides a comprehensive literature review on the synthesis of yolk-shell nanoparticles and applications. In Chapter 3, detailed experimental procedures and characterization techniques employed in this work are presented. In Chapter 4, a volume-confined synthesis of ligand-free gold nanoparticles with tailored sizes for enhanced catalytic activity is discussed. The synthesis of ligand-free Au triangular nanoplate inside a nanocavity of mHSS using photochemical reduction method is introduced in Chapter 5. The following chapter presents the synthesis of ligand-free noble metal M@SiO2 (M=Ag, Au, Pd, Pt) yolk-shell nanoparticles and the growth mechanism. Chapter 7 concludes the core findings of this thesis along with future work. 3 Chapter 2 Chapter 2. Literature Review The syntheses of ligand-free noble metal nanoparticles in this thesis work are achieved by using hollow silica shells as the templates, which lead to a core-shell or yolk-shell structure. Therefore, the current progress for different synthetic strategies of core-shell and yolk-shell nanoparticles (YSNs) as well as their applications is discussed. 2.1 Core-shell nanoparticles Core-shell nanoparticles constitute a special class of nanocomposites materials, in which particles of one material are coated with a thin layer of another material. The coating on the core particles may provide many advantages, viz. an opportunity for surface modification, stability against aggregation, solvent compatibility, controlled release of core, reduction in precious materials and so on.7 Core-shell nanostructures are widely used in different applications such as electronics,8-10 biomedicine,11 pharmaceutical,12 optics,13 and catalysis.1415 Core-shell nanoparticles are broadly divided into five categories as shown in Figure 2.1. Each type of core-shell nanoparticles has its own importance and applications. Based on their material properties, the core-shell nanoparticles are further classified into four different groups: (1) inorganic/inorganic, (2) inorganic/organic, (3) organic/organic and (4) organic/inorganic. Among these, inorganic/inorganic core/shell nanoparticles are the most important class of core-shell nanoparticles, since these types of materials are commonly used as ideal candidates for catalysis, optoelectronics, and bioimaging applications. 4 Chapter 2 Figure 2.1 Different core/shell nanoparticles: (a) spherical core/shell nanoparticles; (b) hexagonal core/shell nanoparticles; (c) multiple small core materials coated by single shell material; (d) nanomatryushka material; (e) movable core within hollow shell material. Reprinted with permission from reference.7 2.2 Rattle–type hollow structures (Yolk-shell / Nanorattles) Rattle–type hollow structures represent a new class of special core-shell nanoparticles and generally referred to as hollow shells with a void space between the solid particle core and the shell, where the core can move freely inside the shell. These are also called yolk-shell nanoparticles (YSNs).16 Recently, researchers have paid much attention towards the synthesis of YSNs, owing to their unique optical and electrical properties and great potential in biosensors, lithium-ion batteries, biomedicine, surface-enhanced Raman scattering, imaging, and catalysis applications.17-19 In particular, they can act as nanoreactors that can provide small spaces for the controlled synthesis of new nanomaterial or for catalytic reactions to 5 Chapter 2 occur. It has been demonstrated that YSNs are ideal candidates for catalytic reactions compared to core-shell nanoparticles. In this section we mainly emphasize on the fabrication of YSNs with metal core of Au, Ag, Pt and Pd and silica shell (SiO2). Figure 2.2 Schematic illustration of etching strategies for preparing YSNs. Reprinted with permission from reference.20 2.3 Methodologies for the fabrication of noble metal (M@SiO2) YSNs 2.3.1 Synthetic approaches Till date, several methods such as (1) selective etching or dissolution method; (2) soft templating method; (3) template free method; (4) galvanic replacement method; (5) pre-shell / ship-in-bottle method, and (6) one-pot method have been employed for the synthesis of YSNs. Among them, selective etching or dissolution methods, pre-shell/post-core, template 6 Chapter 2 free method and one-pot method are noteworthy for the fabrication of different types of YSNs. The combinations of the above methods can also allow to fabricate complex nanorattles with unique properties.20 2.3.2 Selective etching or dissolution method Selective etching methods are commonly employed for the fabrication of YSNs. It is a multi-step process in which the pre-synthesized core materials are first coated with one or two layers of different materials to form sandwich structures followed by a selective etching of one of the coated layers or metal core either by dissolution using a solvent or calcination (Figure 2.2). This method is also employed for the fabrication of YSNs with non-spherical structures such as ellipsoids, sword-in-sheath and cocoons.16 Kim and coworkers21 first applied this approach for the synthesis of Au@carbon nanorattles. Later, Xia and coworkers have successfully synthesized hollow Au@polymer nanoratlle structures. In their work, Au@silica nanoparticles were synthesized using sol-gel method followed by encapsulation with a polymer poly(benzyl methacrylate) (PBzMA) to form Au@SiO2@PBzMA hybrid particles. The silica layer was selectively etched with HF to form hollow polymer beads containing movable gold cores (Figure 2.3).22 7 Chapter 2 Figure 2.3 (A, B) Backscattering SEM and (C, D) TEM images of Au@SiO2@PBzMA particles before (A, C) and after (B, D) HF etching. (E, F) TEM images of Au@SiO2@PBzMA synthesized at different time periods of polymerization: (E) 3 h and (F) 6 h. Reprinted with permission from reference.22 Zhang and coworkers used a similar approach for the synthesis of Au nanorattles. In their method, PS-co-P4VP microsphere polymers were employed as the template. AuCl4- ions were immobilized on the microspheres through coordination affinity of metal precursor ions with microsphere shell, followed by reduction with NaBH4 to form gold nanoparticles on the microspheres. Silica shell was deposited on the spheres through sol-gel process. Finally these hybrid particles were calcinated at high temperature (550 ºC) to remove microsphere templates and eventually Au@microsphere silica shells were obtained (Figure 2.4).23 8 Chapter 2 Figure 2.4 TEM images of (A) PS-co-P4VP microspheres, (B) Au/PS-co-P4VP microspheres, (C, D) Au/PS-co-P4VP@HMSM microspheres, and (E, F) Au@HMSM yolkshell microspheres. Reprinted with permission from reference.23 A.Archer and coworkers24 have also successfully synthesized nanorattles with multicore nanoparticles (Figure 2.5). In this method, Au nanoparticles were grown on the surface of amino-group-functionalized polystyrene (PS) spheres followed by direct coating of silica shell and removal of the PS spheres by calcination at 450 °C. 9 Chapter 2 Figure 2.5 TEM images of (A) Au-decorated PS nanospheres, (B) PS@Au@silica colloids, and (C) hollow silica spheres with multi Au nanoparticles after burning off PS templates. (D) Backscattering SEM (inset, normal SEM) image of multicore hollow silica spheres. Reprinted with permission from reference.24 Kim and coworkers25 used the same strategy to fabricate α-Fe2O3 capsules with Au, Pt, Ag or bimetallic (AuPt) cores. In their method, polyelectrolyte multilayer’s (PEMs) were coated on melamine-formaldehyde (MF) templates by LBL technique and metal nanoparticles were consecutively synthesized on the templates. After loading metallic nanoparticles, the PEMs particles were encapsulated with a layer of α-FeOOH followed by calcination at 700 °C which eventually gave Au@-Fe2O3 nanorattles. This calcination step 10 Chapter 2 not only removed the templates and organic components but also led to the formation of a single large metallic core through thermal agglomeration. Recently, Tang and coworkers26 employed a different approach for the synthesis of YSNs. In their method, a three-layer sandwich structure of organic-inorganic hybrid solid silica spheres (HSSs) was prepared by hydrolysis and condensation of TEOS (tetra ethyl orthosilicate) and N-3-(trimethoxysilyl) propyl ethylenediamine (TSD). The YSNs were fabricated from HSSs by selectively etching the middle layer (organosilica layer) with an appropriate amount of aqueous hydrofluoric acid (HF). Partial removal of the core from the core-shell was demonstrated by Fuertes and coworkers. In their approach, solid core/mesoporous shell silica microspheres were first synthesized followed by loading with a carbon precursor that was sequentially carbonized. Later, controlled dissolution of silica was done with NaOH to produce silica@carbon YSNs with various particle sizes by varying the etching time.27 Shi and coworkers followed a similar strategy for the formation of Au@mSiO2 and Fe2O3@mSiO2 YSNs based on the selective etching of structurally different silica shells. In their method, pre-synthesized Au nanoparticles were coated with a thick silica shell and then coated with a mesoporous silica shell (Figure 2.6).28 The inner thicker silica was selectively removed after treating with Na2CO3 aqueous solution, leaving Au@mSiO2 YSNs. Song and coworkers fabricated Au@SiO2 yolk-shell nanoparticles based on selective etching of Au cores. In their method, presynthesized gold nanoparticles were coated with a silica shell using a modified Stober’s method followed by selective etching of Au cores with KCN solution. Gold cores with different sizes were obtained (Figure 2.7).18 Based on the 11 Chapter 2 similar strategy, a series of rattle type nanostructures such as Pt@carbon,17 Au@ZrO2,29 and Au@SnO230 were fabricated. Figure 2.6 (a) Schematic illustration for the synthesis of Au@mSiO2 and Fe2O3@mSiO2 YSNs; TEM images of (b) Au@SiO2@mSiO2, (c) rattle-type Au@mSiO2, (d and inset) ellipsoidal Fe2O3@SiO2@mSiO2, (e, inset: SEM image of deliberately selected broken ellipsoids) rattle-type Fe2O3@mSiO2. Reprinted with permission from reference.28 Though a number of YSNs were fabricated through this method, the involvement of tedious multi-step process, usage of toxic agents such as HF, KCN for etching and difficulty 12 Chapter 2 in controlling the size, shape and composition of metal core have limited its usage for the fabrication of YSNs. Figure 2.7 (a-f) TEM images of Au@SiO2 nanoreactors and (g, h) silica hollow shells. (a, b) Gold core diameters are 104±9 nm, (c, d) 67±8 nm, and (e, f) 43±7 nm. The scale bars represent 200 nm (a, c, e, and g) and 100 nm (b, d, f, h). Reprinted with permission from reference.18 13 Chapter 2 2.3.3 Pre-shell method or Ship-in-bottle method An alternative approach for the synthesis of nanorattles is a preshell/postcore method, using mesoporous shell themselves as a nanoreactor for the production of tunable sizes of nanoparticles inside the nanocavity of mesoporous shells. The schematic process for the formation of YSNs is shown in Figure 2.8. Briefly, shells were synthesized first and followed by diffusion of two types of reactants such as precursors for the core metal and reducing agents into the nanocavity of the shells. A large number of different types of metal cores are formed inside the shell through self-assembly and chemical reaction. Figure 2.8 Schematic procedure for the preparation of yolk/shell nanoparticles through the ship-in-bottle method. Reprinted with permission from reference.20 By using this approach, Hah and coworkers successfully synthesized Cu@silica rattles. Presynthesized silica shells were soaked in a mixture of copper nitrate and hydrazine monohydrate. They claimed that Cu2+ ions and hydrazine molecules were diffused inside the nanocavity of silica shell and Cu2+ ions were reduced by hydrazine molecules and eventually 14 Chapter 2 Cu@SiO2 nanorattles were formed. The size of copper nanoparticle can be tuned by repeating the soaking-reduction-separation cycle as shown in Figure 2.9.31 Figure 2.9 TEM images of hollow silica particles (a) and Cu core rattle-type silica particles (b: 1 cycle; c: 2 cycles; d: 3 cycles). Reprinted with permission from reference.31 A similar approach was followed by Sara and coworkers for the synthesis of Au@silica nanocomposite in which a gold precursor and NaBH4 were diffused inside the hollow silica shells to form gold nanoparticles inside the hollow silica shell.32 In all the reported methods, normally two types of reactants diffuse sequentially into the cavity and react to form metal nanoparticles. Hence, these approaches have inherent difficulty to ensure that the reaction takes place exclusively inside the shell.16 Cheng and coworkers developed a novel method for the production of various sized Ag nanoparticles inside the polypyrrole-chitosan hollow nanospheres by treating with ultraviolet rays. Under ultraviolet rays the photolysis of chitosan takes place and hydroxymethyl radicals were generated and utilized to reduce the silver ions 15 Chapter 2 to metallic silver in the hollow nanospheres. Moreover, the amount of AgNO3 loaded into the nanoreactors can be easily controlled by adjusting the pH sensitive permeability of the polymer shells. This is schematically represented in Figure 2.10.33 Figure 2.10 Schematic procedure for the preparation of PPy-CS hollow nanospheres containing movable Ag Cores (Ag@PPy-CS). Reprinted with permission from reference.33 Tang and coworkers recently developed a novel nanoreactor for the preparation of tunable gold cores inside the silica nanorattles. They constructed a three-layer sandwich structure in which the middle layer is selectively etched and leaving a plenty of alkyamino groups that act as in situ reducing agent and stabilizer for the growth of metal core. The size of the gold nanoparticles can be tuned by soaking the silica shells in different concentrations of gold precursors. TEM images of silica nanorattles are shown in Figure 2.11.34 But the method requires silica shells with special functional groups. 16 Chapter 2 Figure 2.11 TEM images of (a) silica nanorattles of 110 nm, (b) SRG-1, (c) SRG-2, and (d) SRG-3. Reprinted with permission from reference.34 In recent studies, Lou and coworkers successfully synthesized hollow SnO2 nanoparticles inside the mesoporous silica shell. In their method, mesoporous silica shells (mHSS) were synthesized first and soaked in a molten metal salt hydrate (highly concentrated) with a melting temperature below 100 ºC. The infiltrated mHSS were isolated and calcinated at high a temperature of 700 ºC.35 The infiltrated precursor underwent oxidation process and led to the formation of SnO2 nanoparticle. Finally, mHSS was etched with HF. The schematic process for this method is shown in Figure 2.12. Figure 2.12 Schematic formation of SnO2 hollow spheres inside mesoporous silica nanoreactors. Reprinted with permission from reference.35 17 Chapter 2 2.3.4 Template free methods In this approach, template is not required for the synthesis of YSNs and generally fabrication process is based on Ostwald ripening or Kirkendall effect. 2.3.4.1 Kirkendall diffusion method In this method, a void space is generated at the interface of two different materials owing to varied inter-diffusion rates in a bulk diffusion couple, which leads to a net flow in one direction and is balanced by a flux of vacancies. Based on this method, Yin et al. demonstrated the formation of hollow CoSe nanocrystals of various sizes ranging from 10 to 20 nm as shown in Figure 2.13.36 following this concept, Xu and coworkers successfully synthesized FePt@CoS2 yolk-shell nanocrystals using FePt nanoparticles as seeds.37 Figure 2.13. TEM images showing the formation of hollow CoSe nanocrystals, from top-left to bottom-right: 0 s, 10 s, 20 s, 1 min, 2 min, and 30 min. The Co/Se molar ratio was 1:1. Reprinted with permission from reference.36 18 Chapter 2 2.3.4.2 Ostwald ripening method Ostwald ripening generally refers to the recrystallization process in the solution phase. Zeng and coworkers38 employed this concept for the synthesis of hollow anatase TiO2 nanospheres. Till date, a series of yolk-shell nanoparticles such as TiO2, SnO2, Cu2O, ZnO and ZnS have been fabricated either by using symmetric or asymmetric Ostwald ripening.20 2.3.5 Galvanic replacement method Galvanic replacement reactions are also widely employed for the synthesis of YSNs. Using this technique, Xia’s group and others have fabricated hollow nanostructures from various metals such as Au, Ag and Pd in both aqueous and organic solutions.39-41 Recently, Zeng et al. have fabricated necklace like chains of hollow noble metal nanoparticles (Au, Pt and Pd) using PVP as capping agent.42 Huang and coworkers also fabricated YSNs with an alloy core of less than 20 nm, using the galvanic replacement reaction between Au@AgNPs and HAuCl4 in organic solvent.43 2.3.6 One pot method Recently, one-pot method has been employed for the synthesis of YSNs, where a metal precursor is combined with a mixture of inorganic and organic solvents followed by calcination at high temperature for the formation of YSNs. He and coworkers used this strategy and proposed a general method for the formation of Au, Pt, Pd and Ag metal core nanorattles. In their method, polyacrylic acid (PA) was used both as the template and the reducing agent in the reaction. The metal precursors with PA were mixed in a mixture of dodecyltrimethoxysilane (C12TMS), ethanol and NaBH4 followed 19 Chapter 2 by a calcination at higher temperature 550 ºC (Figure 2.14).44 Though this is a simple method, the use of different organic and inorganic solvents in the synthesis of metal cores may drastically reduce the catalytic activity of the resulting YSNs. Figure 2.14 TEM images of Au@HSNs synthesized using 1 ml of HAuCl4 solution: (a-b) before calcination and after being washed several times with water; scale bars: 50 and 20 nm, respectively; (c-d) after calcination, the small clusters of Au aggregate to give larger Au 20 Chapter 2 nanoparticles (indicated by white arrows); scale bars: 100 and 20 nm, respectively. Reprinted with permission from reference.44 21 Chapter 2 2.4 Applications of yolk-shell noble metal nanoparticles Yolk-shell nanostructures possess many unique properties, such as low density, highsurface to volume ratio, movable core, free space between core and shell, functional tailorabilty, low coefficients of thermal expansion and refractive index. These properties make them ideal candidates for various applications ranging from catalyst support, antireflection surface coatings, sensors, surface-enhanced Raman scattering (SERS) and rechargeable batteries. In addition, these YSNs can also be used as carry vehicles for drug delivery and biomedical imaging agents, owing to their capacity for loading and surface tailor ability with different biomaterials such as fluorescent markers, aptamers, therapeutics and contrast agents. 2.4.1 Yolk-shell nanoparticles as nanoreactors Nanoreactor can be defined as a miniaturized reaction container, which contains catalysts inside. In general, nanoreactors should possess three characteristics: (1) inner and outer diffusion of reactants and products should take place simultaneously; (2) the catalytic species are protected from harsh condition; and (3) they are stable. YSNs exhibit all the abovementioned characteristics and can be used as an ideal candidate for nanoreactors. 45 For example, the catalytic core particles present inside the container are protected by an external shell and prevent them from escaping to the exterior and simultaneously, the reactants and products can diffuse freely through the shells. The core material can move freely inside the container and act as an active catalyst, which can enhance the catalytic activity. The nanospace present between the core and the shell can also provide a homogeneous environment for the reaction (Figure 2.15A and B).20 22 Chapter 2 Figure 2.15 (A) Structural illustration of an Au@oxide composite nanoreactor. (B) TEM image of Au@oxide composite nanoreactors. (C) Typical reactions tested for Au@oxide composite nanoreactors. Reprinted with permission from reference.20 Yolk-shell nanoreactors with gold cores have been fabricated and used as catalysts for reduction of p-nitrophenol, 2-nitroaniline, oxidation of aerobic alcohol and CO oxidation as shown in Figure 2.15C.18, 34, 46-48 For example, Song and coworkers prepared Au@SiO2 YSNs for catalytic reduction of p-nitrophenol (Figure 2.16). Briefly, they synthesized silica-coated 120-nm gold nanoparticles; and the inner core gold nanoparticles were etched to desired sizes using potassium cyanide. It is found that the catalytic reduction of p-nitrophenol exhibited a size dependent property.18 Tang and coworkers34 also successfully demonstrated the use of Au@SiO2 nanoreactors in the catalytic reduction of 2-nitroaniline to phenylenediamine. 23 produce Chapter 2 Apart from using yolk-shell with a single Au core as nanoreactors, multi-core Au-based yolk-shell nanoparticles were also employed as nanoreactors. For example, a multi-core Au@poly(styrene-co-methylacrylic acid) (PS-co-PMAA) nanoreactors have been fabricated and successfully demonstrated as nanoreactors for the aerobic alcohol oxidation to form acid.23 Interestingly, Au@hollow titania or zirconia nanoreactors were also fabricated for efficient catalyst using the presynthesized monodisperse silica coated gold nanoparticles as templates. For example, Schuth and coworkers29 fabricated Au@ZrO2 YSNs with a porous zirconia shell as nanocatalysts for CO oxidation. Stucy and coworkers further synthesized smaller gold nanoparticles inside zirconia or titania shells (Figure 2.17).49 It was found that these Au@ZrO2 yolk-shell nanocatalysts exhibit surprisingly high activity in CO oxidation. The catalytic activity of these nanoreactors can be enhanced by doping with small amount of TiO2 nanoparticles during synthesis.50 In addition to Au@SiO2 YSNs, other metal or metal oxide yolk-shell nanoparticles such as Pt,51 Pd,52-53 Ag31, 54 have been fabricated and used as nanoreactors. For example, Pd@meso-SiO2 YSNs have been used as nanoreactors for the Suzuki cross-coupling reactions55 and showed much enhanced catalytic activity (Figure 2.18). 24 Chapter 2 Figure 2.16 (A) TEM images of Au@SiO2 yolk-shell nanoreactors, and (B) Time-dependent UV-vis spectral changes of Au@SiO2 yolk/shell nanoreactors used for catalytic studies. Reprinted with permission from reference.46 Figure 2.17 (A) TEM images of (a) Au@SiO2, (b) Au@SiO2@ZrO2, (c) Au@hm-ZrO2, (d) Au@SiO2@TiO2, and (e) Au@hm-TiO2. (B) A comparison of catalytic activity for CO 25 Chapter 2 oxidation by Au@hm-TiO2 (squares) and Au@hm-ZrO2 (circles) nanoreactors calcinated at 100 °C and 300 °C respectively. Reprinted with permission from reference.49 Figure 2.18 (A) Schematic illustration for the synthesis of Pd@mesoporous silica composite nanoreactors. (B) TEM images of Pd@mesoporous silica composite nanoreactors. (C) Typical Suzuki reaction. Reprinted with permission from reference.20 2.4.2 Yolk-shell nanoparticles as drug delivery carriers YSNs can be considered as ideal candidates for drug delivery vehicles, owing to special properties such as biocompatibility, hollow space, controllable size and high stability. The inner core and outer shell of YSNs are compatible for modification and functionalization with required organic polymers such as polyethylene glycol (PEG).20 After further functionalizing with folic acid, these YSNs can selectively target and kill cancer cells as shown in Figure 26 Chapter 2 2.19. For example, Yu and coworkers56 have fabricated Co@Au YSNs and successfully employed for nonviral gene transfer vehicles (Figure 2.20). Zhu et al.57 fabricated Fe2O3@silica YSNs and used as carry vehicles for Doxorubicin (DOX) and showed a drug loading of 302 mg/g. A very high drug loading of ibuprofen (IBU) can also be achieved with these nanoreactors. In addition, Fe2O3@silica YSNs can also be used as a contrasting agent for magnetic resonance imaging (MRI) both in vitro and in vivo. Alternatively, the drug releasing properties can be controlled efficiently by coating these nanoreactors with a thermally responsive PEO-PPO-PEO polymer (poly(ethyleneoxide)-poly(propylene oxide)poly(ethylene oxide), known as pluronic and trigging with magnetic response.58 Veres and coworkers successfully fabricated silica shell with inner core of Fe3O4-Au dumbbell-like nanoparticles, which exhibit magnetic, plasmonics and fluorescent properties.59 These nanoreactors are anticipated to be ideal candidates in various fields of targeted delivery by loading with other guest molecules such as genes, drugs, protein and nucleic acids. Figure 2.19 (A) Schematic illustration for the synthesis of YSNs-PEG/FA. (B) A possible mechanism accounting for killing of MCF-7 cells by DOX-YSNs. Reprinted with permission from reference.20 27 Chapter 2 Figure 2.20 TEM images of (a) Co@Au nanospheres and (b) cells exposed to Co@Au nanospheres. Reprinted with permission from reference.56 2.4.3 Yolk-shell nanoparticles for lithium-ion batteries YSNs are also extensively used in the lithium-ion batteries. These nanoreactors provide a confined space to accommodate nanoparticles (NPs) with different volume variation during the charge/discharge cycling, which can efficiently increase the capacity retention properties of the batteries.20 Sn@C YSNs particles are most commonly used as anode material for lithium ion batteries with high capacity and good cycling performance. Lou and coworkers 60 successfully fabricated the SnO2@Carbon nanoreactors and demonstrated that these particles exhibited a higher capacity and better cycling performance, when compared to pure SnO 2 NPs. Apart from this, V2O5-SnO2 double shelled,61 Fe2O3@SnO2,62 and Co3O4 with single, double and triple-shell structures63 were also fabricated and employed for lithium-ion battery electrodes. 28 Chapter 2 In addition to the above applications, the YSNs can also employed for SERS substrates,64 sensors,65-66 solar cells,67 and fuel cells.68 For example, Haes and coworkers64 successfully demonstrated the use of Au-void-silica YSNs for the reproducible SERS substrates. 2.5 Our Proposed Method Despite the large body of work that has been reported for the fabrication of rattle-type nanostructures with mesoporous silica shells, challenges still remain in controlling the size, shape and composition of the enclosed nanoparticles without the assistance of capping ligands. Therefore, we propose an alternative approach for fabricating such nanostructures via a preshell/postcore method. In this approach, mesoporous hollow nanostructures are employed as nanoreactors to allow the diffusion of noble metal precursor solutions into their hollow interior space. Upon thermal decomposition or photochemical reduction, the precursor solution trapped within the nanoreactors will be converted into nanoparticles. Compared to existing strategies, this method is advantageous in the following aspects: 1) This synthetic approach allows facile control over the size of the nanoparticles. This can be simply achieved by varying the concentration of the precursor solution or the volume of the void within the hollow shells; 2) In addition to nanoparticles composed of a single metal, alloyed nanoparticle of two or multiple metals can be easily formed by infiltration of precursor solutions containing multiple noble metal ions; 3) Shape control of the nanoparticles can be achieved by adding trace amount of foreign ions such as Ag+ which will selectively adsorb onto certain facets of the growing nanoparticles; 29 Chapter 2 4) Hybrid nanostructures with each hollow shell containing multiple nanoparticles of different metals may be fabricated by subsequent infiltration of additional precursor solutions once one nanoparticle has been formed inside each of the nanoreactors; 5) Most importantly, the control over size, shape and composition of the nanoparticles can be realized without the use of organic capping ligands that may have adverse effect on their catalytic applications. 30 Chapter 3 Chapter 3. Experimental Section 3.1 Method and materials All chemicals were used as received without further purification. The information about the chemicals and materials used in this thesis is given in Table 3.1. Table 3.1 Chemicals and materials Chemicals Purity Deionized (DI) water 18.2 MΩ·cm Ethanol HPLC grade Fisher Ammonia solution (NH3.H2O) 28-30% Merck Vinyltrimethoxysilane (VTMS) 98% Sigma–Aldrich diameters 124 ± 5 and 266 ± 7 5 wt.% aqueous Microparticles nm (denoted as PS-124 and PS- suspension GmbH, Berlin Sodium borohydride (NaBH4) 99.0% Aldrich Silver nitrate (AgNO3) 99.0% Merck 4-nitrophenol 98.0% Aldrich 99.9% Sigma-Aldrich Polystyrene beads Supplier with 266) Gold(III) chloride trihydrate (HAuCl4∙3H2O) 31 Chapter 3 Trisodium citrate dehydrate 99.9% Sigma-Aldrich Hydrofluoric acid (HF) 48.0% Merck 98.0% Sigma-Aldrich 99.0% Aldrich Sodium tetrachloropalladate (Na2PdCl4) Chloroplatinic acid hydrate . (H2PtCl6 xH2O) Nitric acid (HNO3) Sigma-Aldrich Sodium hydroxide (NaOH) ≥98.0% Merck Sodium chloride (NaCl) 99.5% Sigma-Aldrich 3.2 Solution preparation All solutions were prepared freshly by dissolving chemicals with specific solvents before use. 3.3 Procedures 3.3.1 Synthesis of mesoporous hollow silica shells of size 100 nm and 230 nm (mHSS-100 & mHSS-230) A modified Stober’s method was employed for the coating of vinyl-functionalized silica on PS beads. In a typical procedure, 0.35 mL of VTMS was added in 6.65 mL of H2O under vigorous magnetic stirring for 30 min to form a transparent solution. Simultaneously, 15 mL of 0.5 wt.% PS-124 aqueous dispersion was mixed with 0.88 mL of ammonia under magnetic 32 Chapter 3 stirring for 15 min. To this mixture, the VTMS solution was added drop wise and the reaction was stirred for 6 hours. The resulting PS beads coated with silica shells were separated by centrifugation at 9000 rpm for 15 min, followed by washing with ethanol and water for three times. Finally, the particles (PS-124@SiO2) were dispersed in 10 mL of H2O. For the synthesis of PS-266@SiO2, the same procedure was used, except that the volumes of VTMS, H2O, PS-266 beads (0.5 wt.%), and ammonia were 0.1 mL, 2.21 mL, 5 mL, and 0.29 mL, respectively. 5 mL PS-124@SiO2 or PS-266@SiO2 was transferred into a glass petridish, followed by calcination in a muffle furnace. The vinyl-silica-coated PS spheres were heated to 450 ºC (ramp rate: 2 ºC/min) for 10 hours in air before they were slowly cooled down to room temperature to form mesoporous hollow silica shells (mHSS). The as-obtained mHSS with sizes of 100 nm (mHSS-100) and 230 nm (mHSS-230) were dispersed in water under sonication. As synthesized mesoporous silica shells were used as nano-containers for the synthesis of YSNs by using different novel synthetic methods in the current research. 3.3.2 Synthesis of silica spheres (SiO2) 0.35 mL of VTMS was added in 6.65 mL of H2O under vigorous magnetic stirring at 900 rpm for a period of 30 min. During the stirring process, the organic droplets were completely dissolved and a transparent solution was obtained. Simultaneously, 0.88 mL of ammonia is mixed with 15 mL of water under magnetic stirring for 30 min. Then, the VTMS solution was added to ammonia and water mixture solution drop wise using micropipette and the reaction was further stirred at 900 rpm for a period of 6 hours at room temperature. As obtained SiO2 33 Chapter 3 spheres were calcinated as similar way used for the synthesis of mHSS as described in the above paragraph. 3.3.3 Synthesis of ligand-free Au@SiO2 nanorattles by thermal method The gold precursor, chloroauric acid in water solution, was first impregnated into mHSS230 as follows: 0.3 ml of mHSS (0.3 mg/mL) was centrifuged at 9000 rpm for 10 min. After removing the supernatant, the hollow silica shells were re-dispersed in 500 uL of HAuCl4 solution (with concentrations of 0.5, 0.25, 0.1, 0.01, or 0.005 M). After 24 hours, the hollow silica shells were precipitated out by centrifugation at 9000 rpm for 10 min. The supernatant was removed completely and the silica shells were collected and rinsed with 0.1 mL water by vortex for 1 min to remove excess gold precursor present on the outer surface of the hollow shells. Afterwards, the silica shells impregnated with HAuCl4 solution were transferred onto a glass substrate. The glass substrate was then heated to 250 ºC in an oven at a rate of 20 ºC/min. After 45 min, the sample was cooled to room temperature. The resulting hollow silica shells with gold nanoparticles were collected from the glass substrate by adding 1 mL of water with sonication for 5 min, followed by centrifugation at 9000 rpm for 10 min. The final product, Au nanoparticles within hollow silica shells (Au@SiO2), was dispersed in 0.1 mL of water. Au core with diameter of 7, 10, 26, 36 and 42 nm were obtained with 0.005, 0.01, 0.1 0.25 and 0.5 M HAuCl4 solutions and denoted as Au@SiO2-7, Au@SiO2-10, Au@SiO2-26 Au@SiO2-36 and Au@SiO2-42 respectively. For the synthesis of Au nanoparticles inside 100-nm hollow silica shells (mHSS-100), the same procedure was used except that the concentrations of HAuCl4 were 0.01, 0.25 and 0.5 M; and the resultant Au cores with 34 Chapter 3 diameter of 6, 14 and 18 nm are denoted as Au@SiO2-6, Au@SiO2-14, and Au@SiO2-26, respectively. The synthesis of 26- and 10-nm Au nanoparticles capped with sodium citrate (Au@SC26 and Au@SC-10) was based on reported methods.69-70 The resulting Au nanoparticles were collected by centrifugation and washed with water twice to remove excess sodium citrate. 3.3.4 Catalytic reduction of 4-nitrophenol by Au@SiO2 nanorattles The catalytic reduction reactions were carried out using a standard quartz cuvette with a 1-cm path length. An excess amount of NaBH4 solution (1 mL, 50 mM) was mixed with 50 uL of 2 mM 4-nitrophenol solution in a quartz cell. To this mixture, 25 uL of Au@SiO2 was added. The UV-vis absorption spectra of the solution were recorded. Inductively coupled plasma mass spectrometry (ICP-MS) measurements were conducted by dissolving the samples in fresh aqua regia to determine the amounts of Au in the samples. For all catalytic reactions, the number of gold nanoparticles was kept at a constant (3 × 109). 35 Chapter 3 Table 3.2 Concentration of Au present in different YSNs measured by ICP-MS. Sample name Au concentration (M) Au@SiO2-36 2.9 × 10-4 Au@SiO2-26 1.9 × 10-4 Au@SiO2-10 7.3 × 10-5 2.53 × 10-4 Au@SC-26 2.2 × 10-4 Au@SC-10 3.4 Synthesis of ligand-free Au nanoplates by photochemical reduction method 3.4.1 Synthesis of spherical gold yolk-shell nanoparticles in absence of Ag+ ions (Au@mHSS) 250 uL of mHSS (0.3 mg/mL) were centrifuged at 10,000 rpm for a period of 10 min. After removing the supernatant, the pellet was redispersed in a mixture of 250 uL of 0.1 M HAuCl4 + 5 uL ethanol solution using sonication and vortex for a period of 1min. The solution was allowed to sit for a period of 12 hours to ensure complete diffusion of gold precursor inside the mesoporous silica shells. The mHSS impregnated with gold precursor and ethanol was isolated by centrifugation at 10,000 rpm for 10 min. The supernatant was removed completely and the pellet was collected in a 1.5 mL microcentrifuge tubes and 36 Chapter 3 irradiated under UV light of λ = 254 nm in a black box. The samples were irradiated under UV light for different time periods such as 1, 6, 12 and 24 hours to study the growth of Au nanoparticle inside the cavity of mHSS. After irradiation with UV light for a particular timeperiod, Au@mHSS yolk-shell nanoparticles were washed with 1 mL of water for one time and redispersed in 0.1 mL water with sonication. 3.4.2 Synthesis of Au triangular nanoplates in the presence of Ag+ ions The synthesis process is similar to the method used for the synthesis of spherical Au@mHSS yolk-shell nanoparticles except that the pellet was redispersed in a mixture of 250 uL of 0.1 M HAuCl4 (prepared in saturated NaCl solution) + 5 uL ethanol + 50 uL AgNO3 aqueous solution of different concentrations such as 0.01 M, and 0.0166 M to maintain molar ratio of [AuCl4-]:[Ag+] as 50:1 and 30:1, respectively. The samples were irradiated under UV light of λ = 254 nm for different time periods. After irradiation, Au triangular nanoplates were washed with 1 mL of water for one time and redispersed in 0.1 mL water with sonication. 3.4.3 Etching of Au triangular yolk-shell nanoplates with HF The as-synthesized Au triangular yolk-shell nanoplates as described in the above paragraph were isolated by centrifugation and soaked in 500 uL of 0.1 M PVP for a period of 12 hours. Afterwards, 2 mL of HF (48%) was then added to the sample. The mixture was incubated at 60 °C in a water bath for 30 minutes. After etching, the sample was washed with 1 mL of water for two times and redispersed in 0.1 mL water with sonication. 37 Chapter 3 3.5 Synthesis of ligand-free M@SiO2 (M= Au, Ag, Pt and Pd) nanorattles by using mHSS as smart nanoreactors 3.5.1 Synthesis of Ag@mHSS yolk-shell nanoparticles 250 uL of mHSS (0.3 mg/mL) were centrifuged at 10,000 rpm for a period of 10 min. After removing the supernatant, the pellet was redispersed in 500 uL of 0.1 M AgNO3 solution using sonication for 1min. The pH of the 0.1 M AgNO3 solution is around 5.3. mHSS were soaked in silver nitrate solution for different time periods such as 1 min, 1, 6, 12 and 24 hrs, respectively at room temperature. After soaking, the resulting Ag@mHSS yolkshell nanoparticles were isolated from silver nitrate solution by centrifugation at 10,000 rpm for 10 min and washed with 1 mL of water and redispersed in 0.1 mL of water. 3.5.2 Synthesis of Ag@mHSS yolk-shell nanoparticles with different pH values The synthesis process is similar to the one used for the synthesis of Ag@mHSS yolkshell nanoparticles as described above, except that the pH of 0.1 M silver nitrate solution was adjusted to 1.6, 2.3, 3 and 4.2 by adding HNO3; and for higher pH of 9.3 by addition adding ammonia to the solution. 3.5.3 Synthesis of Au@mHSS yolk-shell nanoparticles 250 uL of mHSS (0.3 mg/mL) were centrifuged at 10,000 rpm for a period of 10 min. After removing the supernatant, the pellet was redispersed in 500 uL of 0.1 mM HAuCl4 solution via sonication for 1min. The pH of 0.1 mM HAuCl4 solution is around 4.4. This solution was incubated at 60 ºC in a water bath for a period of 24 hours. Au@mHSS yolk38 Chapter 3 shell nanoparticles were isolated from the aqueous chloroauric solution by centrifugation at 10,000 rpm for a period of 10 min and washed with 1 mL of water and redispersed in 0.1 mL of water using sonication. 3.5.4 Synthesis of Pd@mHSS yolk-shell nanoparticles The synthesis is similar to the method used for Au@mHSS yolk-shell nanoparticles except that mHSS were incubated in 1 mM Na2PdCl4 solution for a period of 12 hours. The pH of 1 mM Na2PdCl4 solution was 4.4. 3.5.5 Synthesis of Pt@mHSS yolk-shell nanoparticles The synthesis is similar Pd@mHSS yolk-shell nanoparticles except that mHSS were incubated in 500 uL of 1 mM H2PtCl6 solution with a pH of 5 at 80 ºC for a period of 36 hours. The pH of 1 mM H2PtCl6 solution is 3.3. It was adjusted to 5 with NaOH solution. 3.6 Characterization Methods 3.6.1 Ultraviolet-visible spectrophotometer (UV-Vis) UV-vis spectra were recorded using a Shimadzu UV-1601 spectrometer with quartz cuvettes of 1 cm path length at room temperature. 3.6.2 X-ray photoelectron spectroscopy (XPS) The XPS spectra were obtained using AXIS HIS (Kratos Analytical Ltd., U.K.) with an Al-Kα X-ray source (1486.71 eV protons), operated at 15 kV and 10 mA. 39 Chapter 3 3.6.3 Inductively coupled plasma mass spectrometry (ICP-MS) The metal concentrations of samples were obtained based on ICP-MS measurements on an Agilent 7500A. All samples were prepared freshly by dissolving nanoparticles using aqua regia. The resultant solution was diluted using DI water. 3.6.4 Brunauer-Emmett-Teller (BET) measurements for mHSS-100 and mHSS-230 The BET surface areas and pore size distributions of mHSS were analyzed by using a Nova-3000 Series, Quanta chrome nitrogen adsorption apparatus. The samples were degassed for a period of 6 hours at 150 °C prior to nitrogen adsorption measurements. BET multipoint method was used for the determination of BET surface area by using the adsorption data in the relative pressure (P/Po) range of 0.05-0.3. The pore volume and average pore size of the sample were determined by taking the nitrogen adsorption volume at the relative pressure (P/Po) of 0.994. The obtained isotherms are combination of type 2 and type 4 isotherms. The presence of hysteresis in the isotherms indicates the mesoporous nature of the hollow silica nanospheres. Properties of the mHSS-100 are as follows: BET surface area: 332.7 m2/g, average pore size: 3.8 nm, pore volume: 1.321 cm3/g, whereas for mHSS-230 are 140.6 m2/g, 7.3 nm, 0.215 cm3/g respectively. The relatively large pore sizes should be sufficient for easy access of reactants to diffuse into the silica shells. 3.6.5 Zeta-Potential measurements mHSS were dispersed in specific pH solutions and a required volume was taken in a Zeta-potential cuvette and measurements were recorded using Nano ZS- ZEN 3600. 40 Chapter 3 3.6.6 FT-IR measurement for mHSS-100 A required amount of mHSS-100 was mixed with KBr powder and FT-IR spectra were obtained using Shimadzu FT-IR. 3.6.7 Mass spectrum analysis of different noble metal precursor’s solutions The samples were prepared freshly by dissolving respective noble metal precursors in DI water and measurements were recorded using Brucker microtof-Q. 3.6.8 Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) images were obtained using scanning electron microscope (FESEM) operating at 10 kV. The samples were prepared by dropping few drops of resultant nanostructure solutions on clean glass substrate followed by a platinum coating using platinum coater (JEOL JFC-1300) operating at 10 mV for 30 Sec. 3.6.9 Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images and energy dispersive X-ray spectroscopy (EDX) spectra were acquired using JEOL 2010 and JEOL 2100F operating at 200 kV. The samples were prepared by dropping diluted aqueous solutions of the synthesized nanoparticles onto TEM copper grids. 41 Chapter 4 Chapter 4. Volume-confined Synthesis of Ligand-free Gold Nanoparticles with Tailored Sizes for Enhanced Catalytic Activity 4.1 Introduction Metal nanoparticles have shown much enhanced catalytic properties compared to their bulk counterparts.71-73 For example, bulk gold generally is inactive, but Au nanoparticles (NPs) have shown remarkable catalytic activity towards CO oxidation at lower temperatures.74 In recent years, the use of Au NPs as catalyst has increased tremendously.18, 75-79 However, while bare Au nanoparticles are highly active, they aggregate easily, resulting in decreased active surface area and degraded catalytic activity.77, 80-81 Therefore, a protection layer such as inorganic coating is necessary to prevent Au NPs from aggregation.77, 81-82 But a dense coating of inorganic layer hinders the accessibility of reactants to the nanoparticle surface.81, 77 To overcome this issue, Au NPs with yolk-shell or rattle-like structures are proposed. For this type of structures, Au nanoparticles are encapsulated in hollow mesoporous shells with a gap formed between the nanoparticle and the shell.16, 20, 79 On the one hand, the shells can prevent sintering of the metal cores, keeping them isolated even under harsh reaction conditions; on the other hand, the gap between the nanoparticle and the shell maximize the exposed active sites of the nanoparticle. In addition, the porous shells allow the effective diffusion of reactants and products.16, 20, 79 All these properties make yolkshell nanoparticles ideal candidates for catalytic reactions.16, 20, 77, 79 To date, Au yolk-shells or nanorattles are mainly synthesized by three different strategies, namely selective etching or dissolution,21-22, 25, 30, 83-84 preshell/postcore,32, 85 and one-pot synthetic methods.23, 44, 86 Selective etching or dissolution method is a multi-step 42 Chapter 4 assembly process in which presynthesized gold nanoparticles are coated with single or multilayer shells, followed by selective etching of shells or presynthesized gold nanoparticles to form Au nanorattles.16 Based on this strategy, several Au nanorattles have been fabricated, such as Au@carbon,21 Au@polymer,22 Au@ZrO2,83 Au@SnO2,30 and Au@silica.18, 25, 84, 87 Besides the complexity of this approach, this multi-step process also involves the use of toxic agents for etching.16, 34 And it is difficult to form Au NPs less than 10 nm.34, 88 This problem can be overcome with pre-shell/postcore method, where presynthesized micro/mesoporous hollow shells are used as nanoreactors.20, 34 For example, Cavaliere-Jaricot and coworkers fabricated Au@SiO2 nanorattles by reducing HAuCl4 with NaBH4 inside presynthesized hollow silica shell.32 Till date, all reported pre-shell methods generally use two types of reactants to diffuse inside the shell to react, which makes it difficult to ensure that the reaction will take place only inside the hollow silica shell, resulting in a less efficient procedure.16 In order to avoid this problem, Tan and coworkers34 fabricated Au nanorattles with a unique presynthesized hollow silica shell containing plenty of alkylamino groups on the inner shell. These alkylamino groups can act as in situ reducing agent for Au precursor so that the reaction occurs only inside the shells. However, the presence of alkylamino groups inside the shells may interfere with the metal cores and affect their catalytic activity. Recently, one-pot method23, 44, 86 has been employed for the synthesis of Au nanorattles, where an Au precursor is combined with a mixture of inorganic and organic solvents calcinated at a high temperature (e.g., 550 ºC) for the formation of Au nanorattles. Though this method is simple, the use of different capping ligands like polymers (polyvinylpyrrolidone), surfactants (dodecylamine) and solvents (acetone or methanol) along with Au precursor during the synthesis may leave chemical residues and capping ligands on Au core, which may reduce the catalytic activity of gold cores.89-90 In addition, the high 43 Chapter 4 reaction temperature may drastically reduce the porosity of the silica shells.18 Therefore, the development of a simple strategy for the synthesis of nanorattles incorporated with ligandfree Au cores with precisely controlled sizes still remains a challenge. Figure 4.1 Schematic illustration for the synthesis of ligand-free Au nanoparticles using hollow mesoporous silica shells. In this chapter, a new synthetic route is introduced and discussed on tuning sizes of ligand-free Au NPs encapsulated within silica shells (Au@SiO2). In this approach, mesoporous hollow silica shells (mHSS) were employed as nano-container in which impregnated aqueous chloroauric acid is converted to Au nanoparticles by a simple heating process as shown in Figure 4.1. The size of Au cores can be tuned easily by soaking mHSS in chloroauric acid solution of different concentrations. The resulting Au nanoparticles with tailored sizes are free of any capping ligand. We further investigated the use of the asprepared Au@SiO2 nanorattles as catalyst for the reduction of 4-nitrophenol with sodium 44 Chapter 4 borohydride. The Au@SiO2 nanorattles showed much enhanced catalytic activity compared to citrate-capped Au nanoparticles. 4.2 Results and discussion 4.2.1 Synthesis of mesoporous hollow silica shells (mHSS) Monodisperse mesoporous hollow silica shells (mHSS) were synthesized using modified Stober’s method.91 To prepare mHSS with inner diameter of 230 nm (mHSS-230), polystyrene (PS) beads with size of 266 nm (PS-266) were employed as templates for the coating of SiO2. The resulting spherical PS-266@SiO2 core-shells have a shell thickness of 17±1.8 nm (Figure 4.2a). After calcination, mesoporous hollow silica spheres with an average diameter of 230 nm were obtained (mHSS-230, Figure 4.2b). BET measurements reveal that mHSS-230 has a specific surface area of 140.7 m2/g with an average pore size of 7.3 nm (Figure 4.3c-d). This relatively large pore size should facilitate easy access of reactants and products without substantial mass transfer resistance to the inner hollow cavity of mHSS.23, 75, 92 Figure 4.2 TEM images of (a) PS-266@SiO2 core-shell particles, (b) 230-nm SiO2 hollow shells, and (c) SEM image of 230-nm SiO2 hollow shells. 45 Chapter 4 Figure 4.3 N2 adsorption-desorption isotherms of (a) mHSS-100 and (c) mHSS-230; and pore size distributions of (b) mHSS-100 and (d) mHSS-230. 4.2.2 Synthesis of Au@SiO2 nanorattles using mHSS-230 The as-synthesized mHSS-230 hollow shells were then transferred into an aqueous solution of chloroauric acid (HAuCl4.3H2O) for the diffusion of AuCl4- ions into the hollow interior of the shells. After the impregnation of Au precursor, the hollow shells were isolated by centrifugation. The excessive precursor present outside and on the surface of mHSS-230 was removed by rinsing with water.92 It has been shown that HAuCl4 decomposes to AuCl3, AuCl and Au by thermal treatments at 100 °C, 160 °C and 200 °C respectively. 93 Therefore, the mHSS-230 with Au precursor solution inside was heated to 250 ºC in air for a period of 45 min to produce Au nanoparticles to form Au@SiO2 nanorattles. The size of Au 46 Chapter 4 nanoparticles inside mHSS-230 can be easily tuned by impregnating HAuCl4 solutions of different concentrations. For 0.005, 0.01, 0.1, 0.25 and 0.5 M HAuCl4 solutions, the resulting Au nanoparticles are 7 ± 1.8, 10 ± 2.2, 26 ± 4.3, 36 ± 3.7 and 42 ± 7.5 nm (denoted as Au@SiO2-7, Au@SiO2-10, Au@SiO2-26, Au@SiO2-36 and Au@SiO2-42), respectively (Figure 4.4b-f). It is clear that each mHSS-230 hollow shell contains a single spherical Au nanoparticle. The calculated diameter of Au nanoparticles based on the cavity volume of mHSS-230 and the concentrations of HAuCl4 for samples Au@SiO2-7, Au@SiO2-10, Au@SiO2-26, Au@SiO2-36 and Au@SiO2-42 are 10, 13, 27, 37 and 46 nm, respectively, consistent with the measured sizes. Based on the fixed cavity volume and hence the volume of the HAuCl4 solution loaded inside the hollow SiO2 shells, the diameter of the resulting Au nanoparticles should increase linearly with [HAuCl4]1/3 ([HAuCl4] is the concentration of the Au precursor). This is consistent with our observation – the plot of particle size vs. [HAuCl4]1/3 fits well with the prediction (Figure 4.5). It is worth noting that because no capping ligands or chemical reducing agents were used in this synthesis, the Au nanoparticles formed inside mHSS should have a bare surface free of any capping ligand. Therefore, the size of the Au nanoparticles is simply controlled by the confined volume of the precursor solution inside the hollow shells. 47 Chapter 4 Figure 4.4 TEM images of (a) 230-nm SiO2 hollow shells and (b-f) Au nanoparticles with diameters of 7 (Au@SiO2-7), 10 (Au@SiO2-10), 26 (Au@SiO2-26), 36 (Au@SiO2-36) and 42 nm (Au@SiO2-42) synthesized inside of the hollow shells using HAuCl4 solutions of 0.005, 0.01, 0.1, 0.25 and 0.5 M, respectively. 48 Chapter 4 Figure 4. 5 Plot of measured and calculated particle diameters vs. [HAuCl4]1/3. 4.2.3 Characterizations of Au@mHSS nanorattles The formation of Au cores inside of mHSS-230 was further confirmed with high resolution TEM (HRTEM) and UV-vis spectroscopy. Figure 4.6a shows a HRTEM image of Au@SiO2-26. Lattice fringes with a d-spacing of 0.24 nm were observed, matching with (111) planes of fcc gold.94 The UV-vis surface plasmon resonance (SPR) spectra of Au@SiO2-10, Au@SiO2-26 and Au@SiO2-36 nanorattles show peak wavelengths of 540, 551 and 563 nm, respectively. The SPR peak of Au@SiO2 nanorattles red-shifts as the size of the Au core increases, consistent with the prediction of Mie theory.34, 44, 95 The composition 49 Chapter 4 of the samples was also analyzed by energy dispersive X-ray (EDX) analysis. As shown in Figures 4.6c-d, the EDX line scan for a single Au core of Au@SiO2-26 sample clearly shows the signal of Au, Si and O but not Cl (Figure 4.7). The absence of Cl from the Au nanoparticles was also confirmed from XPS analysis, which shows the peaks of O, Si, and Au (Figure 4.8). No detectable signal was observed for Cl at binding energies of 199 and 270 eV. This result is expected – as discussed previously, at the reaction temperature of 250 °C, while HAuCl4 decomposes to Au; Cl should have been removed in the form of HCl or Cl2 gases.34 However, it should be noted that we cannot completely rule out the possibility that a trace amount of Cl may still exist on the surface of the Au nanoparticles. Figure 4.6 (a) HRTEM image of a single Au nanoparticle in Au@SiO2-26. (b) UV-vis extinction spectra of Au@SiO2-10, Au@SiO2-26, and Au@SiO2-36 (c, d) EDX line spectrum of a single gold nanoparticle of Au@SiO2-26. 50 Chapter 4 Figure 4.7 EDX spectrum of Au@SiO2-26. Figure 4.8 XPS spectra of as-synthesized Au@SiO2-26: (a) Au 4f and (b) survey scan. 51 Chapter 4 4.2.4 Synthesis of Au@SiO2 nanorattles using mHSS-100 To demonstrate the versatility of our method and the effect of cavity volume of mHSS on tuning the size of Au nanoparticles, we also synthesized Au nanoparticles using mesoporous hollow silica shells with size of 100 nm (mHSS-100, Figure 4.9a-b). By soaking mHSS-100 in HAuCl4 solution with concentrations of 0.01, 0.25, and 0.5 M, Au cores with diameters of 6 ± 1.7 nm, 14 ± 2.8 nm, and 18 ± 3.8 nm were obtained, respectively (Figure 4.9 c, e, g). It is clear that at the same Au precursor concentrations, 0.01, 0.25 and 0.5 M, the sizes of Au nanoparticles formed inside mHSS-100 were reduced by 40%, 61% and 57% (6 vs. 10 nm, 14 vs. 36 nm and 18 vs. 42 nm) compared to those formed in mHSS-230, respectively. This result clearly indicates that the size of Au core can be tuned by either changing the cavity volume of mHSS with the same gold precursor concentration or changing the concentration of gold precursor but with same cavity volume of mHSS. 52 Chapter 4 Figure 4.9 (a) SEM and (b) TEM images of 100-nm SiO2 hollow shells (mHSS-100). (c, e, g) TEM images of Au nanoparticles with diameters of 6 (Au@SiO2-6), 14 (Au@SiO2-14), 18 53 Chapter 4 nm (Au@SiO2-18), synthesized inside of mHSS-100 by impregnating HAuCl4 solutions of 0.01, 0.25 and 0.5 M, respectively; (d, f, h) corresponding histograms of Au nanoparticle sizes for Au@SiO2-6, Au@SiO2-14, and Au@SiO2-18 respectively; (i) a low magnification TEM image of Au@SiO2-14. 4.2.5 Catalytic activities of Au@SiO2 nanorattles for reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in presence of excess NaBH4 The Au nanoparticles formed inside the silica hollow shells have a bare surface free of any capping ligands, an ideal system for the study of catalytic activity of nanoparticles with different sizes without the interference of undesirable species. In this work, the catalytic activity of Au@SiO2 nanorattles was evaluated for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in presence of excess NaBH4 as a model reaction. The reduction kinetics of 4-NP to 4-AP can be monitored easily by UV-vis spectroscopy.23, 96 After mixing 4-NP with NaBH4, the aqueous solution showed an absorption peak at 400 nm, which can be attributed to 4-nitrophenolate ions in alkaline solution.23, 96 In the absence of Au@SiO2 nanorattles, the peak at 400 nm remained unchanged for more than one day, confirming that the reduction will not proceed in absence of Au@SiO2 nanorattles. For samples with the presence of Au@SiO2 nanorattles, the absorbance at 400 nm reduced and a new peak at 300 nm started to appear due to the conversion from 4-NP to 4-AP (Figures 4.10 and 4.11).18, 23, 96 54 Chapter 4 Figure 4.10 Time-dependent UV-Vis absorption spectra for the reduction of 4-nitrophenol catalyzed by (a) Au@SiO2-26 and (c) Au@SC-26, respectively. (b) ln(Ct/C0) vs. time for the reduction of 4-nitrophenol catalyzed with Au@SiO2 and Au@SC nanoparticles of different sizes in the presence of excess NaBH4. (d) Conversion of 4-nitrophenol vs. time (Inset: TOF bar chart). 55 Chapter 4 Figure 4.11 Time-dependent UV-Vis absorption spectra for the reduction of 4-nitrophenol catalyzed by (a) Au@SC-10, (b) Au@SiO2-10, and (c) Au@SiO2-36, respectively. 56 Chapter 4 The concentration of NaBH4 in all the reactions was fixed at 500-times higher that of 4-NP. Therefore, the concentration of NaBH4 can be considered as a constant during the reaction.97 Thus, the reduction rate constant can be calculated based on pseudo-first-order kinetics.18, 23, 86, 96 Figure 4.10b shows the linear plots of ln(Ct/C0) vs. t (Ct and C0 correspond to the concentration of 4-NP at time t and its initial concentration, respectively), which can fit the first-order reaction kinetics. The apparent rate constants (kapp) were calculated based on the slopes of the lines, and the results are shown in Table 1. For the comparison of catalytic activities of Au@SiO2 nanorattles, kapp was normalized to the number of moles of Au atoms (ka,Au) and surface area of Au nanoparticle (ka’,Au) (Table 1). The calculated ka,Au for samples Au@SiO2-36, Au@SiO2-26 and Au@SiO2-10 were 8.2 × 105, 1.3 × 106, and 2.1 × 106, s-1mol-1 whereas the corresponding k a’,Au were 486, 550 and 563 s-1m-2 respectively. The ka,Au and ka’,Au of Au@SiO2-10 is nearly 2.6 and 1.2 times higher than that of Au@SiO2-36. This is expected since the catalytic activity of Au nanoparticles is dependent on size due to the change in specific surface area.23, 86 In addition, Au nanoparticles with smaller size contain higher fraction of low-coordination metal sites at edges and vertices than bigger ones.18, 98 It is noteworthy to state that the ka,Au of Au@SiO2 nanorattles is higher than that of other Au@SiO2 nanorattles reported in the literature.23, 86, 9697 The enhanced catalytic activities of Au@SiO2 nanorattles obtained in this work can be attributed to the absence of capping ligands. In general, the presence of capping ligands on gold nanoparticles may reduce the catalytic activity of Au nanocatalysts by blocking active sites and hindering the access of reactants to the nanoparticle surface.89-90 57 Chapter 4 Table 4.1 Comparison of the catalytic activities of Au@SiO2 nanorattles and sodium citratecapped Au nanoparticles for the reduction of 4- nitrophenol. ka ,Au Sample Au@SiO2-36 Au@SiO2-26 Au@SiO2-10 Au@SC-26 Au@SC-10 Au NPs kapp (nm) (s-1) 36 26 10 26 10 -1 (s m ) 5.93 × 10 -3 3.5 × 10 -3 5.3 × 10 -4 1 × 10 -3 2.5 × 10 -2 ka,Au -1 TOF -1 (s mol ) (h-1) 486 8.2 × 105 118 550 1.3 × 106 178 563 2.1 × 106 260 157 3.7 × 105 60 265 9.8 × 105 130 -4 To demonstrate the effect of capping ligands on the catalytic activity of Au nanocatalysts, we further tested the catalytic activity of Au nanoparticles capped sodium citrate (Au@SC-10 and Au@SC-26 for 10 and 26 nm sodium citrate-capped Au nanoparticles). Au@SC-10 and Au@SC-26 were synthesized based on reported methods (Figure 4.12).69-70 58 Chapter 4 Figure 4.12 TEM images of sodium citrate-capped Au nanoparticles: (a) Au@SC-10, (c) Au@SC-26 and (b, d) the corresponding histograms of Au nanoparticle sizes, respectively. Figure 4.11c shows the time-dependent UV-vis absorption spectra of the reduction of 4-NP catalyzed by Au@SC-26. The resulting ka,Au of Au@SC-10 and Au@SC-26 are only 46% and 28% of that of Au@SiO2-10 and Au@SiO2-26, respectively. The lower catalytic activity of sodium citrate-capped Au nanoparticles can be attributed to the steric hindrance exhibited by sodium citrate molecules on Au nanoparticles.99 The reduction rate of 4-NP depends on the rate of electron transfer from BH4- to 4-NP on the particle surface, the diffusion rate of 4NP to catalyst surface, as well as the diffusion rate of 4-AP away from the metal surface.86, 100 For bare Au nanoparticles enclosed on mHSS shells, 4-nitrophenolate ions with a molecule size around 0.25 nm101 can easily diffuse through the mesoporous shells and adsorb on the 59 Chapter 4 surface of Au cores, where catalytic reaction occurs immediately at a high rate because no steric hindrance exists, thanks to the absence of capping ligands on the surface of gold cores. Whereas for Au@SC particles, the adsorption of 4-NP on gold surface is more difficult due to both the steric97, 99, 102 and electrostatic97, 99 hindrance exhibited by sodium citrate present on gold nanoparticles, leading to much lower reduction rate of 4-NP compared to that of Au@SiO2. Figure 4.10d plots the conversion vs. time for the reaction catalyzed by Au@SiO2 nanorattles and Au@SC particles. Au@SiO2-36 shows a conversion of more than 90% in less than 10 minutes, much faster compared to Au@SC particles (58% in 10 minutes). The turn over frequency (TOF, calculated based on the moles of product formed per molar Au per hour at 90% conversion of 4-NP) of the samples were further calculated and presented in Table 4.1. The TOF of Au@SiO2-10 is nearly 2.5 times higher than that of Au@SiO2-36, while the TOFs of Au@SiO2-26 and Au@SiO2-10 are nearly 3 and 2 times higher than that of Au@SC-26 and Au@SC-10 respectively, indicating the much enhanced catalytic activity of bare Au nanoparticles prepared within mesoporous SiO2 shells. 4.3 Conclusion In summary, we developed a new synthetic method for the synthesis of ligand-free Au nanorattles using mesoporous hollow silica shells (mHSS). By tuning the concentration of aqueous chloroauric acid and size of mHSS, different sizes of gold cores could be obtained with a simple heating process. The as-prepared Au nanorattles exhibited better catalytic activity for the reduction of 4-nitrophenol compared to Au nanoparticles capped with sodium citrate. This method can be extended for the synthesis of other metal nanoparticles to achieve tailored sizes without the assistance of any capping ligands. 60 Chapter 5 Chapter 5. Synthesis of Ligand-free Au Triangular Nanoplates 5.1 Introduction In recent years, much attention has been paid towards the synthesis of Au triangular nanoplates owing to their unique optical properties,103-104 enhanced catalytic activity,105-106 anisotropic electrical conductivity,107 and enhanced electric field108 compared to spherical Au nanoparticles. Various applications have been demonstrated for Au nanoplates in sensing,109 bioimaging,110-111 photothermal therapy,112-113 catalysis,114-115 and biological diagnostics.116117 To date, a number of solution-based synthetic methods have been developed for Au triangular nanoplates including seed-mediated growth,116,118 thermal method,119 photocatalytic approach,120 and biological108 method. For example, Mirkin and coworkers used seed-mediated method for the synthesis of Au triangular nanoplates.118 Periara et al. employed photocatalytic approach by using tin (IV) porphyrin as a photocatalyst for the synthesis of Au triangular nanoplates in solution.120 Current synthetic methods to produce Au triangular nanoplates inevitably involve the use organic molecules or polymers such as polyvinylpyrrolidone (PVP),120 cetyltrimethylammonium bromide (CTAB),121 and other surfactants122 as shape-control agents. Although it is typical to employ organic capping ligands during the growth of Au nanocrystals to achieve different morphologies, the presence of these organic species on the surface of the resulting Au nanocrystals may drastically affect their catalytic activity,123-124 cause instability in harsh conditions,125 and also limit their capability for biological applications.122 Oftentimes, removal or replacement of the capping agents from nanocrystal surface has been a tedious step. It is still a great challenge to grow capping agent-free Au nanocrystals with controlled shape, size, and exposed facets. 61 Chapter 5 In an effort to make ligand-free noble metal nanocrystals, Tang and coworkers used hydrothermal method to synthesize icosahedral Au-Pt alloy nanocrystals inside the nanocavity of hybrid porous silica shells.123 In chapter 4, a facile volume confined method is discussed for tuning the size of naked Au nanoparticles by simply soaking mesoporous hollow silica shells (mHSS) in chloroauric acid solutions of different concentrations followed by a simple heating process. The resulting ligand-free Au nanoparticles showed much enhanced catalytic activity compared to Au nanoparticles capped with citric acid. However, due to the absence of morphology-controlling agent, only spherical Au nanocrystals were produced with this method. In this chapter, a new photochemical reduction method is introduced to control the shape of Au nanoparticle. It was demonstrated that ligand-free Au triangular nanoplates can be synthesized in the presence of Ag+ ions without using any organic capping agents. The synthesis route for tuning the shape of Au nanoparticles is schematically shown in Figure 5.1a. Briefly, mesoporous hollow silica shells were soaked in a mixture of HAuCl4 dissolved in saturated NaCl aqueous solution, AgNO3 aqueous solution, and ethanol. The impregnated mHSS were then exposed to UV light to form Au triangular nanoplates free of any organic capping ligands. For the same synthesis but without using AgNO3, Au nanospheres instead of nanoplates were obtained, indicating the shape-selective effect of Ag+ ions in the synthesis. 62 Chapter 5 Figure 5.1 (a) Schematic illustration for the growth of ligand-free Au triangular nanoplates and nanoparticles inside silica shells under UV irradiation. (b-d) TEM images of Au nanocrystals synthesized under UV irradiation for 24 hours at different ratios of [AuCl4-]: [Ag+] -- (b) 30:1; (c) 50:1; and (d) without Ag+. (e) UV-vis absorption spectra of the Au nanocrystals. 63 Chapter 5 5.2 Results and discussion 5.2.1 Synthesis of Au triangular nanoplate inside mHSS Porous mesoporous hollow silica shells (mHSS) were used as nano-containers for the synthesis of ligand-free Au triangular nanoplates. Properties of the mHSS are as follows: inner diameter: 100 nm; shell thickness: 16 nm, BET surface area: 332.7 m2/g; average pore size: 3.83 nm (Figure 5.2 & 4.3a-b). Figure 5.2 TEM images of (a) 124 nm PS beads, (b) PS-124@vinyl-SiO2, (c) mHSS, and (d) FESEM image of mHSS. The Au nanoplates were formed using a UV-light driven photochemical reduction method with a mixture of HAuCl4 dissolved in saturated NaCl, aqueous AgNO3 solution, and ethanol inside the mHSS. TEM images of the samples show that at an atomic ratio of Au:Ag = 30:1, 64 Chapter 5 the resulting Au nanostructures mainly consist of Au triangular nanoplates with an average edge length of 30 ± 4.4 nm (Figure 5.1b). For reactions at a higher Au to Ag ratio or without Ag+, the samples are mainly Au spherical particles with an average size of 28 ± 4.3 nm (Figures 5.1c-d). The different shapes of the Au nanocrystals can also be confirmed from the UV-vis absorption spectra as shown in Figure 5.1e. For Au triangular plates obtained at a Au:Ag ratio of 30:1, a broad peak centered at 842 nm along with a sharp peak at 537 nm can be observed, consistent with the typical SPR of Au triangular nanoplates.119 5.2.2 Characterizations of Au triangular nanoplates Elemental analysis of the Au triangular nanoplates reveals both Au and Ag with an atomic ratio of Au:Ag = 33:1 (Figure 5.3), close to the molar ratio of HAuCl4 and AgNO3 added in the reaction (30:1). Au Ag Figure 5.3 EDX elemental mappings of Au (red) and Ag (green) and the spectrum for the Au nanoplates synthesized at Au:Ag = 30:1. All scale bars are 50 nm. 65 Chapter 5 To further investigate the morphology of the Au triangular nanoplates formed inside mHSS, we etched the silica shell with HF in the presence of PVP. PVP was added in the etching reaction to ensure that Au nanocrystals can remain dispersed. TEM images of the Au nanostructures after the removal of the silica shell show that the majority of the particles have a triangular shape, although some other shapes including decahedron and octahedron can be also found (Figure 5.4a). The selected area electron diffraction (SAED) pattern of a single Au triangular nanoplate is shown in Figure 5.4b. The indicated spots can be indexes as 1/3{422}, {220}, and 2/3{422} diffractions of Au, respectively.126 HRTEM image of a Au triangular nanoplate shows lattice fringes with a d-spacing of 0.234 nm, which corresponds to (111) plane of Au crystal.127 Figure 5.4d shows TEM image of a side-view of a Au triangular nanoplate with a thickness of ~ 10 nm. The growth of Au nanoplates in the presence of Ag+ inside mHSS was monitored with the reaction time. Figure 5.5 shows the TEM images of Au triangular nanoplates obtained at 1, 6, 12, and 24 hrs, respectively. For a period of 1 hour, most of mHSS are empty without Au nanoparticles (Figure 5.5a). But for 6 hours, mHSS containing Au nanoparticle with an average size of 11 ± 1.6 nm were obtained (Figure 5.5b). As reaction time increased to 12 and 24 hours, a Au triangular nanoplates can be found in each mHSS, with average edge lengths of 23 ± 3.1 and 30 ± 4.4 nm, respectively (Figure 5.5c-d). 66 Chapter 5 Figure 5.4 (a) TEM image of the Au triangular nanoplates after the removal of silica shells. (b) SAED pattern of a Au triangular nanoplate. (c) HRTEM image of a Au triangular nanoplate. (d) TEM side view of a Au triangular nanoplate. 67 Chapter 5 a b c d Figure 5.5 TEM images of Au triangular nanoplates synthesized in the presence of Ag+ ions under UV irradiation at different times: (a) 1, (b) 6, (c) 12 and (d) 24 hrs (Scale bars: 50 nm). The insets are the corresponding HRTEM images (Scale bars: 5 nm). 5.2.3 Synthesis of spherical gold nanoparticles inside mHSS in the absence of Ag+ ions Figure 5.6 shows the TEM images of Au nanoparticles formed inside of the mHSS without using Ag+ ions. The average sizes of Au nanoparticle inside mHSS were 8 ± 1, 20 ± 2.1, 28 ± 4.3 and 30 ± 3.01 nm for 1, 6, 12, and 24 hours of UV irradiation, respectively. The average size of Au nanoparticle increased from 8 ± 1 to 30 ± 3.01 nm inside mHSS as the 68 Chapter 5 duration of UV irradiation increased from 1 hour to 24 hours. This result confirms that Ag+ ions play a key role in assisting the anisotropic growth of Au nanoplates inside mHSS.128-129 Figure 5.6 TEM images of Au@mHSS synthesized without Ag+ ions under UV light for different time periods: (a) 1 hour, (b) 6 hour, (c) 12 hour and (d) 24 hour. (All scale bars are 50 nm). 69 Chapter 5 5.2.4 Effect of chloride ions on the formation of Au triangular nanoplates During the synthesis, NaCl was added to avoid the precipitation of AgCl. To examine the effect of NaCl in the formation of Au triangular nanoplates, control experiment was conducted using a mixture containing NaCl, HAuCl4 and ethanol but without AgNO3 in the mHSS. In this case, only spherical Au nanoparticles were obtained, indicating that NaCl would not change the growth mode of Au nanoparticles (Figure 5.7). Figure 5.7 TEM images of the samples obtained by soaking mHSS in 0.1M aqueous HAuCl4 solution for a period of 24 hours (a) with ethanol but no UV light irradiation and (b) with UV light but without ethanol. (c) TEM image of Au@mHSS synthesized by soaking mHSS in a mixture of 0.1 M HAuCl4 (prepared with saturated NaCl solution) and ethanol but without Ag+ ions. 5.2.5 Proposed mechanism for the growth of Au triangular nanoplate inside mHSS The mechanism for the formation of gold nanoparticle inside mHSS can be explained as follows. When mHSS were soaked in the precursor solution, AuCl4- ions and ethanol 70 Chapter 5 molecules can freely diffuse through the mesopores of mHSS and are loaded inside the cavity. During UV light irradiation, ethanol molecules can generate superoxide and ethoxy radicals130 which can act as reducing agents for the reduction of metal ions and convert AuCl4- ions to Au atoms. The concentration of Au atoms gradually increases and once it reaches supersaturation limit, the nucleation of Au atoms starts the mHSS and eventually grow into nanoparticle inside mHSS. Au nanoparticle were not obtained in mHSS when the experiments were conducted in the absence of ethanol (Figure 5.7b), indicating the key role of ethanol in the reduction of AuCl4- ions. In addition, when the reaction was conducted in the presence of ethanol but without irradiation of UV light, no metal nanoparticle were produced inside the mHSS (Figure 5.7a), which further confirms that ethanol alone cannot act as a reducing agent for the reduction of AuCl4- ions in the absence of UV light. A similar mechanism of UV-light driven generation of radicals for the reduction of AuCl4- ions has been reported in previous studies.128-129 The addition of Ag+ ions in the growth reaction is critical to promote anisotropic growth of Au nanoparticle. The effect of Ag+ ions on the growth of Au nanocrystals has been reported before. For example, Placido et al.128 investigated the key role of Ag+ ions in promoting the growth of Au nanorods in presence of CTAB surfactant using photochemical method. The authors claimed that Ag (0) adsorbs preferably onto {100} and {110} facets promoting the crystal growth along [010] direction of the fcc crystal lattice, which leads to the formation of Au nanorods. Yang and coworkers129 also observed similar results but they claimed that Ag(0) is present only in the initial growth of Au nanorods and its absence in final growth of Au nanorods may be due to re-oxidation of Ag(0) to AgBr in the presence of AuCl4- ions. In our work, we observed mainly Au triangular nanoplates instead of Au 71 Chapter 5 nanorods inside mHSS. This can be attributed to twinned Au seeds formed inside mHSS in the presence of Ag+ ions. HRTEM images for the Au seeds confirmed the formation of twinned or single crystalline seeds in the presence and absence of Ag+ ions, respectively (Figure 5.8). The addition of Ag+ ions can lower the reduction kinetics131-132 of AuCl4- and promote the formation of twinned Au seeds in UV light driven reactions,132 since slower growth of metal atoms preferably forms twinned seeds.123 In addition, the formation of twin plane metal seeds can be enhanced in silver halide system.133, 134 Since the crystallographic structure of the initial Au seeds may dictate the shape of the final nanocrystal,132-133, 135 the presence of twin planes in Au seed may be eventually responsible for the growth of plate-like Au nanocrystals.132, 134 Based on SAED, the presence of 1/3 {422} reflections (Figure 5.5b) which are forbidden in a single crystal fcc metal may be attributed to parallel twin planes that are typically observed in Au and Ag nanoplates.131, 136 The TEM image of a single Au triangular nanoplate in side view shows the presence of a characteristic bright/dark contrast adjacent to the twin domains (Figure 5.5d), which further confirms the presence of a twin plane in Au nanoplate.120 The results are consistent with previous results on formation of Au or Ag nanoplates.108, 120, 131-132 Figure 5.8 HRTEM images of Au seeds obtained in the presence and absence of Ag+ ions: (a) twinned and (b) single crystalline seeds respectively. 72 Chapter 5 5.3 Conclusion In summary, Au triangular nanoplates were successfully grown inside the nanocavity of mesoporous silica shells. The amount of Ag+ ions in the reaction may determine the final shape of Au nanoparticles. Though this method is demonstrated for tuning the shape of Au nanoparticles, it could be extended for controlling the shape of other noble metal nanoparticles such as Pt and Pd which may be influenced by the presence of other metal ions. We believe this research may shed light on the path towards the synthesis of ligand-free anisotropic metal core yolk-shell nanoparticles. . 73 Chapter 6 Chapter 6. Synthesis of M@SiO2 (M = Ag, Au, Pd, Pt) Yolk-Shell Nanoparticles 6.1 Introduction In recent years, noble metal yolk-shell nanoparticles (YSNs) have gained wide attention because of their myriad applications not only as ideal candidates for catalytic reactions16, 20, 77, 79, 137-138 but also as nanoreactors,20, 137 surface-enhanced Raman scattering (SERS),139 drug- delivery carriers,20, 59, 140 and sensors.141 To date, several strategies have been employed for the synthesis of YSNs, among them template free method, selective etching or dissolution method, pre-shell/post-core and one-pot method are noteworthy.16, 20, 137 Template free methods are mainly based on Ostwald ripening or Kirkendall process for the synthesis of yolk-shell nanoparticles.137 By using this strategy several yolk-shell nanoparticles such as Au@TiO2,142 Au@Cu2O,143 Pt@CeO2,51 Pt@Fe2O3,144 and Ag@Fe2O3145 have been fabricated. Though this method is a simple and efficient way to fabricate yolk-shell nanoparticles, it is limited by the specific requirement of shell material with metal nanoparticles to form YSNs.137 Selective etching is a multi-step process in which the presynthesized noble metal nanoparticles are coated with a single or hybrid layer, followed by a selective etching of coated layer or the core metal to form yolk-shell nanoparticles.16 By using this method several YSNs such as Au@SiO2,16, 18, 25, 84 Pt@TiO2,146 Pd@CeO2,147 and Au@SnO2141 are fabricated. However, this method involves tedious multiple steps along with the use of toxic agents for etching.16, 34 These problems could be overcome by using pre- shell/ post core method in which the pre-shell is used as nanoreactors for the synthesis of yolk-shell metal nanoparticles.16, 34 Typically, this approach needs two or multiple reactants 74 Chapter 6 to diffuse inside the pre-shell, making it difficult to ensure that the reaction occurs only in the cavity of the pre-shells.16 To overcome this problem, Tang and coworkers34 proposed a unique pre-shell method by synthesizing a complex three-layer sandwich structure, in which the middle layer was selectively etched. The remaining alkylamino groups can act as in situ reducing agent and stabilizer for the growth of metal core. Recently, one pot method23, 44, 86 has been employed for the fabrication of YSNs. Though this method is simple, the use of different capping ligands and requirement of high calcination temperature in the synthesis process may drastically affect the catalytic activity of YSNs.18, 89-90 Designing a simple and versatile strategy for the fabrication of YSNs still remains a great challenge.20 In chapter 4 & 5, we have introduced volume-confined and photochemical reduction methods for tuning the size and shape of ligand-free gold nanoparticles inside mHSS. In this chapter, we further extend this synthetic approach as a general method to the fabrication of ligand-free noble metal yolk-shell nanoparticles without using any capping and reducing agents. In this work, mHSS were used as smart nanoreactors and soaked in respective metal precursor aqueous solutions for the growth of noble metal yolk-shell nanoparticles. The formation of ligand-free YSNs can be tuned simply by varying the pH of the metal precursor aqueous solution. A critical pH > 4 is required for the formation of YSNs as shown in Figure 6.1. While when the pH < 4, empty mHSS are observed without the formation of YSNs. It is worth noting that the controlled growth of ligand-free nanoparticle inside mHSS by simply varying the pH of the solution has been rarely reported. 75 Chapter 6 Figure 6.1 Schematic illustration for the synthesis of ligand-free yolk-shell nanoparticles M@mHSS (M=Ag, Au, Pd & Pt). 6.2 Results and discussion 6.2.1 Synthesis of ligand-free M@SiO2 nanorattles (M = Ag, Au, Pd & Pt) using mHSS Monodisperse mesoporous hollow silica shells (mHSS) with an inner diameter of 100 nm were used as “smart” nanoreactors for the synthesis of ligand-free M@SiO2 (M = Ag, Au, Pd & Pt) (YSNs). The mHSS, having a shell thickness of 16 ± 1.8 nm and BET surface area and average pore size of 332.7 m2/g and 3.83 nm, respectively, were synthesized by using a modified method as reported in the literature (see section 5.2.1, chapter 5). 91 The YSNs were synthesized by simply soaking the mHSS in respective metal precursor aqueous solutions (AgNO3, HAuCl4, Na2PdCl4 or H2PtCl6) (Figure 6.1). The formation of ligand-free nanoparticles can be tuned by varying the pH of the solutions. Figures 6.2 and 6.3 show the 76 Chapter 6 TEM images of ligand-free M@mHSS (M = Ag, Au, Pd and Pt) yolk-shell nanoparticles. It is clear that each mHSS contains a single movable spherical metal core with average sizes of 15 ± 1.4, 17 ± 4.2 and 12 ± 2.6 nm for Ag@mHSS, Au@mHSS and Pd@mHSS, respectively, whereas polyhedral metal core with an average size of 14 ± 1.9 nm was observed for Pt@mHSS. Figure 6.2 TEM images of ligand-free M@mHSS (M = Ag, Au, Pd & Pt) synthesized by soaking mHSS in respective metal precursor solutions: (a) Ag@mHSS, (b) Au@mHSS, (c) Pd@mHSS, and (d) Pt@mHSS. 77 Chapter 6 Figure 6.3 Low magnification TEM images of (a) Ag@mHSS, (b) Au@mHSS, (c) Pd@mHSS, and (d) Pt@mHSS. 6.2.2 Characterizations of ligand-free M@SiO2 nanorattles (M = Ag, Au, Pd & Pt) The formation of ligand-free metallic core inside mHSS was further confirmed by HRTEM and EDX analysis (Figure 6.4). From HRTEM, the measured lattice fringes with dspacing values of 0.237 nm, 0.24 nm, 0.223 nm, and 0.22 nm correspond to the (111) planes of Ag,148 Au,149 Pd150 and Pt,151 respectively (Figure 6.4a, d, g, j). The compositions of the ligand-free metal cores were analyzed with EDX elemental mappings, which clearly show the presence of Ag, Au, Pd and Pt in addition to Si and O (Figure 6.4c, f, i, l). 78 Chapter 6 Figure 6.4 (a, d, g, j) HRTEM images of Ag, Au, Pd and Pt core inside of mHSS. (b, e, h, k) TEM images of M@mHSS (M= Ag, Au, Pd & Pt) and (c, f, i, l) the corresponding EDX elemental mappings of Ag, Au, Pd, Pt (red), O (blue) and Si (green), respectively. (All unmarked scale bars are 50 nm). 6.3 Growth mechanism for the formation of ligand-free nanorattles 6.3.1 Synthesis of Ag@mHSS nanorattles at different time periods To further understand the growth mechanism for the formation of ligand-free metal core inside mHSS, we monitored the growth of Ag core inside the mHSS for different time periods of soaking in the precursor solution (Figure 6.5). For 1 min soaking, very small Ag 79 Chapter 6 nanoparticle were seen inside the mHSS (Figure 6.5b), whereas the average size of the Ag core increased to 11 ± 1.6, 14 ± 1.9, 15 ± 2.2 and 16 ± 1.4 nm for a soaking time period of 1, 6, 12 and 24 hours, respectively (Figure 6.5c-f). The growth of Ag cores inside mHSS was also examined by UV-vis spectroscopy (Figure 6.6). The UV-vis spectra show a characteristic absorbance peak around 300 nm for silver nitrate aqueous solution152 aged for 24 hours (red line), while mHSS shows no absorbance peak in the spectra. But after infiltrating mHSS with silver nitrate solution for a period of 24 hours, there is an appearance of shoulder around 430 nm which confirms the formation of silver nanoparticles153 inside mHSS. 80 Chapter 6 Figure 6.5 TEM images of Ag@mHSS synthesized by soaking mHSS in 0.1M AgNO 3 solution for different time periods: (a) mHSS, (b) 1 min, (c) 1 hour, (d) 6 hour, (e) 12 hour and (f) 24 hour. 81 Chapter 6 Figure 6.6. UV-vis absorption spectra of mHSS, AgNO3 solution aged for 24 hours, and mHSS in AgNO3 solution aged for 24 hours. 6.3.2 Synthesis of Ag@mHSS nanorattles at different pH values To study the effect of pH on the formation of metal core inside mHSS, we also conducted control experiments by soaking mHSS in silver nitrate aqueous solution with different pH values (Figure 6.7a-f). It was found that the formation of Ag@mHSS yolk-shell nanoparticles takes place only at pH > 4; whereas for pH < 4 there is no Ag core inside 82 Chapter 6 mHSS. Although Ag@mHSS yolk-shell nanoparticles can also be obtained at pH = 9.3 (Figure 6.7f), the etching of silica started to take place.154 Figure 6.7 TEM images of Ag@mHSS synthesized by soaking mHSS in 0.1M AgNO3 solution for a period of 1 hour at different pH values: (a) 1.6, (b) 2.3, (c) 3.1, (d) 4.2, (e) 5.3 and (f) 9.3. 83 Chapter 6 6.3.3 Proposed mechanism for the formation of ligand-free nanorattles The proposed mechanism for the formation of ligand-free metal core inside mHSS is shown in Figure 6.8. mHSS mainly consists of hydroxyl species (≡Si-OH) and siloxane bridges (≡Si-O-Si≡).155-156 As shown in Figure 6.9, the FT-IR spectrum of mHSS with peaks at 946 cm-1 and 1086 cm-1 confirms the presence of silanol and siloxane groups.157 The hydroxyl species can be protonated (≡Si-OH2+) at pH lower than the isoelectric point of the mHSS when the surface of mHSS is positively charged. At pH > 4, deprotonated species (≡Si-O-) are mainly present on the mHSS with a net negative charge.158 To confirm this, the zeta-potentials of mHSS were measured at different pH values (Table 6.1). It is confirmed that the negative charge of mHSS gradually increased with the increase of pH of the solution.159 At high pH, the deprotonated silanol species (≡Si-O-) can act as a relatively strong reducing agent compared to hydroxyl species (≡Si-OH) and may start to reduce metal ions.160 Figure 6.8 Schematic illustration of the formation of metal core nanoparticles inside mHSS. 84 Chapter 6 Figure 6.9 FT-IR spectrum of mHSS. Table 6.1. Zeta-potentials of mHSS at different pH values. Condition Zeta-Potential pH = 1.8 -1.66 pH = 2.6 -2.56 pH = 5.3 -43.1 pH = 11.6 -44.2 85 Chapter 6 For the synthesis of ligand-free yolk-shell nanoparticles, the mHSS were soaked in the respective metal precursor solutions at pH > 4. During soaking, the noble metal precursor ions can diffuse into the mHSS where it can be reduced by ≡Si-O- 160 and converted to metallic atoms. Inside the cavity of mHSS, the concentration of metallic atoms can easily reach supersaturation limit due to the confined space. Therefore, nucleation starts preferably inside the cavity.161-162 With the growth of the metallic core, the metallic atoms are gradually depleted inside the cavity and a concentration gradient is developed, which facilitates the diffusion of the precursor from bulk solution into the cavity and the continuous growth of metallic core nanoparticle. The nucleation preferably takes place in the cavity of mHSS compared to the outer planar/convex surfaces may be attributed to the lower critical radius of nucleation.161, 163 In addition, the formation of metallic nanoparticles in the pores of mHSS is hindered probably due to the absence of deprotonation of hydroxyl species (≡Si-OH) in the pores. Different degrees of deprotonation of hydroxyl species (≡Si-OH) in silica pores have been reported previously in the literature.154 6.3.4 Effect of nanocavity of mHSS on the formation of ligand-free nanorattles To further study the effect of nanocavity for the formation of yolk-shell nanoparticles, control reactions were performed with solid silica spheres (SiO2, average size = 326 nm). By mixing SiO2 spheres with Ag or Au precursor solutions, a large number of small metallic nanoparticles were formed on the SiO2 surface (Figure 6.10). This result confirms that the nanocavity of mHSS plays a vital role in the formation of yolk-shell nanoparticles. 86 Chapter 6 Figure 6.10 TEM images of (a, c) Au and (b, d) Ag nanoparticles formed on the surface of SiO2 spheres. 6.4 Conclusion In summary, we proposed a facile method for the synthesis of ligand-free M@mHSS (M = Ag, Au, Pd and Pt) yolk-shell nanoparticles. The formation of yolk-shell nanoparticles can be tuned by simply controlling the pH of the metallic precursor aqueous solution. Though this method is demonstrated for the synthesis of ligand-free noble metal yolk-shell nanoparticles, it can be extended for the synthesis of other transition metallic nanoparticles such as ruthenium and rhodium yolk-shell nanoparticles. In our previous study (chapter 4), we have demonstrated the uncapped Au@SiO2 yolk-shell nanoparticles exhibits better catalytic activity as compared to metallic nanoparticles functionalized with capping ligands. We 87 Chapter 6 believe these ligand-free yolk-shell nanoparticles may also exhibit better catalytic activity and can be used as ideal candidates for catalytic reactions. 88 Chapter 7 Chapter 7. Conclusions and Future Work 7.1 Conclusions In conclusion, we have successfully designed facile synthetic methods for the synthesis of ligand-free noble metal nanoparticles. Firstly, a volume confined synthetic method was introduced to tune the size of ligand-free Au nanorattles using mHSS. By varying the concentration of aqueous chloroauric acid and the size of mHSS, gold nanoparticles of different sizes were obtained inside the mHSS. The Au nanorattles exhibited enhanced catalytic activity for the reduction of 4-nitrophenol to 4-aminpphenol with NaBH4 compared to Au nanoparticles capped with sodium citrate. We further demonstrated that the shape of Au nanocrystals can be tuned from spheres to triangular nanoplates by adding Ag+ ions in the reaction with a new photochemical reduction method. We also developed a facile method for the synthesis of M@mHSS (M = Ag, Au, Pd and Pt) yolk-shell nanoparticles free of capping ligand. This was achieved by soaking mHSS in respective metal precursor solutions with a pH > 4. The synthesis of ligand-free nanoparticles using mesoporous hollow silica shells has advanced the current understanding for the growth of nanoparticles in nanospace cavities. The core findings of this thesis are summarized as follows: 1. Mesoporous hollow silica shells of size 230 nm (mHSS-230) were used as nanocontainers for the impregnation of HAuCl4 solution before they are separated from the bulk solution. With a simple heating process, the Au precursor confined within the cavity of the isolated hollow shells is converted into ligand-free Au nanoparticles. The size of the Au nanoparticles can be tuned precisely by loading HAuCl4 solution of different concentrations, or by using mHSS with different cavity volumes. For 0.005, 89 Chapter 7 0.01, 0.1, 0.25 and 0.5 M HAuCl4 solutions, the resulting Au nanoparticles are 7 ± 1.8, 10 ± 2.2, 26 ± 4.3, 36 ± 3.7 and 42 ± 7.5 nm (denoted as Au@SiO2-7, Au@SiO210, Au@SiO2-26, Au@SiO2-36 and Au@SiO2-42), respectively. The calculated diameter of Au nanoparticles based on the cavity volume of mHSS-230 and the concentrations of HAuCl4 for samples Au@SiO2-7, Au@SiO2-10, Au@SiO2-26, Au@SiO2-36 and Au@SiO2-42 are 10, 13, 27, 37 and 46 nm, respectively, consistent with the measured sizes. To demonstrate the effect of cavity volume of mHSS on the sizes of Au nanoparticles, we also synthesized Au nanoparticles using 100-nm mesoporous hollow silica shells (mHSS-100). For mHSS-100 soaked in HAuCl4 solutions with concentrations of 0.01, 0.25, and 0.5 M, Au cores with diameters of 6 ± 1.7, 14 ± 2.8, and 18 ± 3.8 nm were obtained, respectively. It is clear that at the same concentrations of Au precursor, 0.01 and 0.25 M, the sizes of Au nanoparticles formed inside mHSS-100 were reduced by 40%, 61% and 57% (6 vs. 10 nm, 14 vs. 36 nm and 18 vs. 42 nm) compared to those formed in mHSS-230, respectively. This result clearly indicates that the size of Au core can be tuned by either changing the cavity volume of mHSS with the same gold precursor concentration or changing the concentration of gold precursor but with same cavity volume of mHSS. The catalytic activity of Au@SiO2 nanorattles was evaluated for the reduction of 4-nitrophenol (4NP) to 4-aminophenol (4-AP) in the presence of excess NaBH4 as a model reaction. The catalytic activity of ligand-free nanorattles is compared with sodium citrate capped nanoparticles (denoted as Au@SC-26, Au@SiO2-10). The calculated apparent rate constant (kapp) was normalized to the number of moles of Au atoms (ka,Au) for samples Au@SiO2-36, Au@SiO2-26 and Au@SiO2-10 were 8.2 × 105, 1.3 × 106, and 2.1 × 106 s-1mol-1, respectively. The ka,Au of Au@SiO2-10 is nearly 2.6 times higher 90 Chapter 7 than that of Au@SiO2-36. The TOF of Au@SiO2-10 (260 h-1) is nearly 2.5 times higher than that of Au@SiO2-36 (118 h-1), while the TOFs of Au@SiO2-26 (178 h-1) and Au@SiO2-10 (260 h-1) are nearly 3 and 2 times higher than that of Au@SC-26 (60 h-1) and Au@SC-10 (130 h-1), respectively, indicating much enhanced catalytic activity of ligand-free Au nanoparticles prepared within mesoporous SiO2 shells. 2. Mesoporous hollow silica shells with inner diameter of 100 nm (mHSS-100) were used as nanoreactors for the synthesis of ligand-free Au triangular nanoplates by UVlight driven photochemical reduction. The shape of the gold nanoparticles was tuned successfully from sphere to triangular nanoplate by varying the molar ratio of [AuCl4]:[Ag+] in the reaction. As the molar ratio of [AuCl4-]:[Ag+] increased in the reaction, a higher percentage of Au triangular nanoplates were obtained inside mHSS. For a molar ratio 30:1, nearly all mHSS contain Au triangular nanoplates with an average edge length of 30 ± 4.4 nm. The presence of twin planes in Au seed may be responsible for plate-like growth of Au nanoplates. For a reaction time of 1 hour, most of mHSS were empty without Au nanoparticles. But for 6 hours, Au nanoparticles with an average size of 11 ± 1.6 nm were formed in mHSS. As the time period of the reaction increased to 12 and 24 hours, Au triangular nanoplates with an average edge length 23 ± 3.1 and 30 ± 4.4 nm, respectively, can be found in most mHSS. But in absence of Ag+ ions, the average size of Au core inside mHSS was 8 ± 1, 20 ± 2.1, 28 ± 4.3 and 30 ± 3.01 nm for different time period of UV irradiation such as 1, 6, 12 and 24 hours respectively. All the mHSS contain a single movable spherical gold nanoparticle inside mHSS, which further confirms that Ag+ ions play a key role in assisting the anisotropic growth of Au cores inside mHSS. The mechanism for the formation of gold triangular plates inside mHSS may be as follows: AuCl4- ions, Ag+ 91 Chapter 7 ions and ethanol molecules can freely diffuse into the cavity of the mHSS. Under UV light, superoxide and ethoxy radicals can be generated from ethanol and act as reducing agents for the reduction of metal ions. In presence of Ag+ ions, the growth of the Au takes place and 3. A simple method was developed for the fabrication of ligand-free noble metal M@SiO2 (M=Ag, Au, Pd, Pt) yolk-shell nanoparticles (YSNs) without using any capping and reducing agents. The ligand-free YSNs were synthesized by simply soaking the mHSS in respective metal precursor aqueous solutions (AgNO3, HAuCl4, Na2PdCl4 and H2PtCl6). mHSS contain a single movable spherical metal core with an average size of 15 ± 1.4, 17 ± 4.2 and 12 ± 2.6 nm for Ag@mHSS, Au@mHSS and Pd@mHSS, respectively, whereas polyhedral metal core with an average size of 14 ± 1.9 nm was observed for Pt@mHSS. The formation of YSNs can be tuned simply by varying the pH of the respective metal precursor aqueous solution. It was found that the formation of YSNs takes place only for higher pH > 4; whereas, for pH < 4 there was no formation of metal core inside mHSS. This reason can be as follows: at higher pH > 4, deprotonated species (≡Si-O-) are present on the mHSS. Therefore, the noble metal precursor that diffuses inside the mHSS is reduced by ≡Si-O- and converted to metallic atoms. The nucleation preferably takes place in the cavity of mHSS compared to the outer planar/convex surfaces possibly due to the lower critical energy of nucleation inside mHSS. The nanocavity of mHSS plays a critical role in enhanced nucleation rate inside the cavity which eventually leads to the formation of yolk-shell nanoparticles. 92 Chapter 7 7.2 Future Work Based on the results presented in this thesis, some potential areas related to ligand-free NMNs for future investigations are highlighted below. 1. We demonstrated that ligand-free Au@SiO2 nanorattles exhibit enhanced catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol in presence of excess NaBH4 compared to sodium citrate capped Au nanoparticles (see chapter 4). But in fact, nanoreactors containing alloyed nanoparticles often exhibit better physical and chemical properties such as high catalytic activity and excellent structural stability at high temperatures compared to monometallic particles. Different sizes and shapes of alloyed metal nanoparticles exhibit superior catalytic performance. For example, AuAg@mHSS showed a high CO conversion rate at 50 ºC.77 The volume confined method developed for tuning different sizes of Au nanorattles (see chapter 4) can be extended for the fabrication of nanorattles with tunable composition of alloyed nanoparticles inside mHSS. In addition, further investigations can be conducted towards different catalytic applications of ligand free YSNs (see section 2.4, chapter 2). For example, palladium nanoparticles are considered to be efficient catalysts for Suzuki cross-coupling reactions.164 Chen et al. demonstrated the catalytic applications of Pd@mHSS in Suzuki cross-coupling reactions. We can employ this method to examine the catalytic activity of as-synthesized ligand-free Pd@mHSS nanorattles (see chapter 6). We believe these YSNs will show higher catalytic efficiency owing to absence of capping agents on their surface. 2. Till date, all the reported YSNs mainly contain a single metal core impregnated inside mHSS (see chapter 2). In order to improve catalytic activity and to achieve unique 93 Chapter 7 properties, YSNs with two or more distinct individual metallic nanoparticles are needed. It is still a great challenge to fabricate YSNs containing distinct multi-core nanoparticles. The novel methods proposed in this thesis can be extended for the synthesis of multi-core YSNs to further investigate the cooperative effect and their usage as nanocatalysts for different reactions. 3. The catalytic and optical properties of YSNs can be effectively tailored by tuning their size and shape. Till date, all the reported YSNs mainly contain spherical metal core nanoparticle inside mHSS. We successfully tuned the shape of Au nanoparticle from spherical to triangular nanoplate inside the cavity of mHSS by addition of Ag+ ions in photochemical reduction method (see chapter 5). This method can be extended for tuning the shapes of other noble metal nanoparticle such as Pt and Pd nanoparticles. In addition, one can explore the effect of combination of different ions (Ag+, Cu2+, PtCl62-, and PdCl42-) and reaction conditions for the synthesis of anisotropic metal core YSNs. 4. It has been shown that Au-Pt alloyed nanoparticles impregnated in SnO2 shells act as ideal candidates for electrocatalyst applications. Recently, Lou and coworkers35 have successfully synthesized hollow SnO2 shells using SiO2 nanoreactors by calcination method. They used highly concentrated SnCl2·2H2O molten liquid as precursor for the synthesis of SnO2 hollow shells at 700 °C. The volume confined method discussed in chapter 4, can be extended for the synthesis of Au-Pt nanodendrites or Au-Pt alloy hybrid nanoparticles in SnO2 shells by impregnating Au-Pt@mHSS in concentrated SnCl2·2H2O precursor and subsequent removal of outer silica shell by HF etching. We believe this ligand-free hybrid materials can exhibit superior electrocatalyst activity. 94 Chapter 7 5. In recent years much attention has been paid towards the synthesis of ruthenium (Ru) and rhodium (Rh) nanoparticles owing to their unique catalytic properties.165-166 The novel methods discussed in this thesis (see chapter 5 & 6) can be extended for the synthesis of ligand-free Ru and Rh YSNs. 6. It has been shown that Fe3O4@mHSS and Au@mHSS nanorattles act as potential candidates for drug delivery vehicles and theranostic agents in biomedical applications. As-synthesized YSNs discussed in this thesis can be extended for applications such as nanomedicine, contrasting and image probing molecules and theronastic agents. In addition, these methods can be extended for the synthesis of Fe3O4@mHSS YSNs. 7. The internal, chemical and electrical properties of silica shells can be modified by doping with small quantity of impurities like carbon, nitrogen and dye molecules. 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Phys.Chem Lett., 2015 (Under Review) Firdoz Shaik, Weiqing Zhang, Wenxin Niu, Xianmao Lu, “A New Universal Method: Synthesis of M@SiO2 (M = Ag, Au, Pd, Pt) Yolk-Shell Nanoparticles” (Under Review) Zhipheng Sun, Shaik Firdoz, Esther Ying-Xuan Yap, Lan Li, Xianmao Lu “Hierarchically structured MnO2 nanowires supported on hollow Ni dendrites for high-performance supercapacitors ” Nanoscale 2013, 5, 4379-4387 Wenxin Niu, Weiqing Zhang, Shaik Firdoz, Xianmao Lu, “Dodecahedral Gold Nanocrystals: The Missing Platonic Shape” J. Am. Chem. Soc., 2014, 136, 3010-3012 Wenxin Niu, Weiqing Zhang, Shaik Firdoz, Xianmao Lu, “Controlled Synthesis of Palladium Concave Nanocubes with Sub-10-nm Edges and Corners for Tunable Plasmonic Property” Chemistry of Materials, 2014, 26, 2180–2186 Weiqing Zhang, Johnny, Shaik Firdoz, Xianmao Lu, “Growth of Au@Ag Core-shell Pentatwinned Nanorods: Tuning the End Facets”, Chem. Eur. J., 2013, 19, 12732–12738 J Y Sun, Z. K. Wang, H. S. Lim, V. L. Zhang, S. C. Ng, M. H. Kuok, W. Zhang, S. Firdoz, 115 Annexure X. M. Lu, “Brillouin study of confined eigenvibrations of silver nanocubes”, Solid State Commun., 2012, 152, 501-503 116 [...]... investigated a facile method for the synthesis of ligand- free noble metal M@SiO2 (M=Ag, Au, Pd, Pt) yolk-shell nanoparticles In this study, we demonstrated that the nanocavity of mHSS plays a critical role to lower the critical radius of the nucleation and enhanced nucleation rate inside the cavity of mHSS for the formation of yolk-shell nanoparticles The formation of ligand- free YSNs can be tuned simply... the templates to form nanoparticles inside their cavity without using any capping ligands The detailed research activities and scope of the thesis are as follows: A facile volume confined synthesis was developed for tuning the size of ligand- free Au nanoparticles In this method, mesoporous hollow silica shells were employed as nanoreactors for tuning size and shape of noble metal nanostructures mHSS... required for the synthesis of ligand- free Ag, Pd, Pt, as well as Au nanoparticles, while at pH 4, deprotonated species (≡Si-O-) present on the mHSS surface can act as reducing agent for the conversion of noble metal precursor ions into metal atoms The nanocavity of mHSS also plays a critical role to lower the critical radius of nucleation for the... the nanocavity of mHSS The effect of Ag+ and ethanol for the formation of Au triangular nanoplates was examined Finally, we extended the synthesis of ligand- free nanoparticles to other noble metals including Ag, Pd, Pt which cannot be obtained inside of mHSS by using simple heating methods In this study, the precursor solutions of different noble metals were loaded inside ix Summary the mHSS first The... synthesize ligand- free noble metal nanoparticles However, besides tedious procedures, the difficulties in scaling up the synthesis and controlling the growth of nanoparticles have 1 Chapter 1 limited the use of these methods In addition, bare NMNs are highly active and aggregate easily 1.2 Objectives The current research objective is to develop synthetic methods for the growth of ligandfree NMNs with... thesis along with future work 3 Chapter 2 Chapter 2 Literature Review The syntheses of ligand- free noble metal nanoparticles in this thesis work are achieved by using hollow silica shells as the templates, which lead to a core-shell or yolk-shell structure Therefore, the current progress for different synthetic strategies of core-shell and yolk-shell nanoparticles (YSNs) as well as their applications... YSNs can be tuned simply by varying the pH of the noble metal precursor aqueous solution We found that pH > 4 is required for the synthesis of yolk-shell nanoparticles; whereas, for pH < 4 nanoparticles were not obtained This is because the deprotonated species (≡Si-O-) on mHSS surface at higher pH > 4 can act as reducing agents for the conversion of noble metal ions to nanoparticle 1.3 Organization of... ligand- free gold nanoparticles with tailored sizes for enhanced catalytic activity is discussed The synthesis of ligand- free Au triangular nanoplate inside a nanocavity of mHSS using photochemical reduction method is introduced in Chapter 5 The following chapter presents the synthesis of ligand- free noble metal M@SiO2 (M=Ag, Au, Pd, Pt) yolk-shell nanoparticles and the growth mechanism Chapter 7 concludes the... crystalline seeds respectively 72 xviii List of Figures Figure 6.1 Schematic illustration for the synthesis of ligand- free yolk-shell nanoparticles M@mHSS (M=Ag, Au, Pd & Pt) 76 Figure 6.2 TEM images of ligand- free M@mHSS (M = Ag, Au, Pd & Pt) synthesized by soaking mHSS in respective metal precursor solutions: (a) Ag@mHSS, (b) Au@mHSS, (c) Pd@mHSS, and (d) Pt@mHSS 77... spaces for the controlled synthesis of new nanomaterial or for catalytic reactions to 5 Chapter 2 occur It has been demonstrated that YSNs are ideal candidates for catalytic reactions compared to core-shell nanoparticles In this section we mainly emphasize on the fabrication of YSNs with metal core of Au, Ag, Pt and Pd and silica shell (SiO2) Figure 2.2 Schematic illustration of etching strategies for